In vitro methods of detecting cells sensitive to sweet tastants by detecting expression of hetero-oligomeric T1R2/T1R3 taste receptors

ABSTRACT

The present invention relates to the discovery that the T1R receptors assemble to form functional taste receptors. Particularly, it has been discovered that co-expression of T1R1 and T1R3 results in a taste receptor that responds to umami taste stimuli, including monosodium glutamate. Also, it has been discovered that co-expression of the T1R2 and T1R3 receptors results in a taste receptor that responds to sweet taste stimuli including naturally occurring and artificial sweeteners. 
     Also the present invention relates to the use of hetero-oligomeric taste receptors comprising T1R1/T1R3 and T1R2/T1R3 in assays to identify compounds that respectively respond to umami taste stimuli and sweet taste stimuli. 
     Further, the invention relates to the constitutive of cell lines that stably or transiently co-express a combination of T1R1 and T1R3; or T1R2 and T1R3; under constitutive or inducible conditions. The use of these cells lines in cell-based assays to identify umami and sweet taste modulatory compounds is also provided, particularly high throughput screening assays that detect receptor activity by use of fluorometric imaging. 
     Finally, the invention relates to the discovery that some compounds, e.g., lactisole, inhibit both the activities of human T1R2/T1R3 and T1R1/T1R3 receptors, and accordingly the sweet and umami taste, suggesting that these receptors may be the only sweet and umami receptors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 12/270,813, filed Nov.13, 2008 now U.S. Pat. No. 8,153,763, which is a divisional of U.S. Ser.No. 11/939,433, filed Nov. 13, 2007, now abandoned, which is acontinuation of U.S. Ser. No. 10/725,488, filed Dec. 3, 2003, now U.S.Pat. No. 7,303,886, which is a divisional of U.S. Ser. No. 10/179,373,filed Jun. 26, 2002, now U.S. Pat. No. 7,368,285, which is acontinuation-in-part of U.S. Ser. No. 10/035,045, filed Jan. 3, 2002,now U.S. Pat. No. 7,241,880, U.S. Ser. No. 09/897,427, filed Jul. 3,2001, now U.S. Pat. No. 6,955,887, and U.S. Ser. No. 09/799,629, filedMar. 7, 2001, now U.S. Pat. No. 7,244,835; U.S. Ser. No. 10/179,373additionally claims priority to U.S. Provisional Application Ser. No.60/300,434, filed Jun. 26, 2001, U.S. Provisional Application Ser. No.60/304,749, filed Jul. 13, 2001, U.S. Provisional Application Ser. No.60/310,493, filed Aug. 8, 2001, U.S. Provisional Application Ser. No.60/331,771, filed Nov. 21, 2001, U.S. Provisional Application Ser. No.60/339,472, filed Dec. 14, 2001, U.S. Provisional Application Ser. No.60/372,090, filed Apr. 15, 2002, and U.S. Provisional Application Ser.No. 60/374,143, filed Apr. 22, 2002, all of which are incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention in part relates to the discovery that the T1Rreceptors assemble to form functional taste receptors. Particularly, ithas been discovered that co-expression of T1R1 and T1R3 results in ataste receptor that responds to umami taste stimuli, includingmonosodium glutamate. Also, it has been discovered that co-expression ofthe T1R2 and T1R3 receptors results in a taste receptor that responds tosweet taste stimuli including naturally occurring and artificialsweeteners.

Also the present invention relates to the use of hetero-oligomeric tastereceptors comprising T1R1/T1R3 and T1R2/T1R3 in assays to identifycompounds that respectively respond to umami taste stimuli and sweettaste stimuli.

Further, the invention relates to the construction of cell lines thatstably or transiently co-express a combination of T1R1 and T1R3; or T1R2and T1R3; under constitutive or inducible conditions.

The use of these cell lines in cell-based assays to identify umami andsweet taste modulatory compounds is also provided, particularly highthroughput screening assays that detect receptor activity by the use offluorometric imaging.

2. Description of the Related Art

The taste system provides sensory information about the chemicalcomposition of the external world. Mammals are believed to have at leastfive basic taste modalities: sweet, bitter, sour, salty, and umami. See,e.g., Kawamura et al., Introduction to Umami: A Basic Taste (1987);Kinnamon et al., Ann. Rev. Physiol., 54:715-31 (1992); Lindemann,Physiol. Rev., 76:718-66 (1996); Stewart et al., Am. J. Physiol.,272:1-26(1997). Each taste modality is thought to be mediated by adistinct protein receptor or receptors that are expressed in tastereceptor cells found on the surface of the tongue (Lindemann, Physol.Rev. 76:718-716 (1996)). The taste receptors that recognize bitter,sweet, and umami taste stimuli belong to the G-protein-coupled receptor(GPCR) superfamily (Hoon et al., Cell 96:451 (1999); Adler et al., Cell100:693 (2000)). (Other taste modalities are believed to be mediated byion channels.)

G protein-coupled receptors mediate many other physiological functions,such as endocrine function, exocrine function, heart rate, lipolysis,and carbohydrate metabolism. The biochemical analysis and molecularcloning of a number of such receptors has revealed many basic principlesregarding the function of these receptors. For example, U.S. Pat. No.5,691,188 describes how upon a ligand binding to a GPCR, the receptorundergoes a conformational change leading to activation of aheterotrimeric G protein by promoting the displacement of bound GDP byGTP on the surface of the Gα subunit and subsequent dissociation of theGα subunit from the Gβ and Gγ subunits. The free Gα subunits and Gβγcomplexes activate downstream elements of a variety of signaltransduction pathways.

This invention relates to the three-member T1R class of taste-specificGPCRs. Previously, the T1R receptors were hypothesized to function assweet taste receptors (Hoon et al., Cell 96:541-51 (1999); Kitagawa etal., Biochem Biophys Res. Commun. 283:236-42 (2001); Max et al., Nat.Genet. 28:58-63 (2001); Montmayeur et al., Nat. Neurosci. 4: 412-8(2001); Sainz et al., J. Neurochem. 77: 896-903 (2001)), and Nelson etal. (2001) have recently demonstrated that rat T1R2 and T1R3 act incombination to recognize sweet taste stimuli. The present inventionrelates to two discoveries. First, as is the case for rat T1R2/T1R3,human T1R2 and T1R3 act in combination to recognize sweet taste stimuli.Second, human T1R1 and T1R3 act in combination to recognize umami tastestimuli. Therefore, T1R2/T1R3 is likely to function as a sweet tastereceptor and T1R1/T1R3 is likely to function as an umami taste receptorin mammals. The likely explanation for the functional co-dependence ofT1R1 and T1R3 and the function co-dependence of T1R2 and T1R3 is that,like the structurally related GABA_(B) receptor (Jones et al., Nature396: 5316-22 (1998); Kaupmann et al., Nature 396: 683-7 (1998); White etal., Nature 396:679-82 (1998); Kuner et al., Science 283: 74-77 (1999)),T1Rs function as heterodimeric complexes. However, it is alternativelypossible that this functional co-dependence reflects a necessary buttransient interaction that ultimately produces functionally independentmonomeric or homomultimeric taste receptors.

The identification of characterization of taste receptors which functionas sweet and umami receptors is significant as it will facilitate theuse of these receptors in assays for identifying compounds that modulate(enhance or block) sweet and umami taste. These compounds would beuseful for improving the taste and palatability of foods, beverages,medicinals for human or animal consumption. Particularly, an assay thatutilizes a functional sweet receptor would allow the identification ofnovel sweeteners.

SUMMARY OF THE INVENTION

The present invention relates to the discovery that differentcombinations of T1Rs, when co-expressed, produce functional tastereceptors that respond to taste stimuli. Particularly, the presentinvention relates to the discovery that co-expression of T1R2 and T1R3results in a hetero-oligomeric taste receptor that responds to sweettaste stimuli. Also, the present invention relates to the discovery thatthe co-expression of T1R1 and T1R3 results in a hetero-oligomeric tastereceptor that responds to umami taste stimuli such as monosodiumglutamate.

The present invention also relates to cell lines that co-express T1R1and T1R3, preferably human, or T1R2 and T1R3, preferably human. Inpreferred embodiments these cell lines will express elevated amounts ofthe receptors, either constitutively or inducibly. These cell linesinclude cells that transiently or stably express T1R1 and T1R3 or T1R2and T1R3.

Also, the present invention provides assays, preferably high throughputscreening assays, that utilize the T1R2/T1R3 taste receptor, or theT1R1/T1R3 receptor, preferably high throughput cell-based assays, toidentify compounds that modulate sweet or umami taste. The inventionalso provides assays that include taste tests to confirm that thesecompounds modulate sweet or umami taste.

OBJECTIONS OF THE INVENTION

Toward that end, it is an object of the invention to provide a family ofmammalian G protein-coupled receptors, herein referred to as T1Rs, thatmediate taste perception.

It is another object of the invention to provide fragments and variantsof such T1Rs that retain activity, e.g., that are activated by and/orbind sweet or umami taste stimuli.

It is yet another object of the invention to provide nucleic acidsequences or molecules that encode such T1Rs, fragments, or variantsthereof.

It is still another object of the invention to provide expressionvectors that include nucleic acid sequences that encode such T1Rs, orfragments or variants thereof, which are operably linked to at least oneregulatory sequence such as a promoter, enhancer, or other sequenceinvolved in positive or negative gene transcription and/or translation,and/or protein export.

It is still another object of the invention to provide human ornon-human cells, e.g., mammalian, yeast, worm, or insect cells, thatfunctionally express at least one of such T1Rs, or fragments or variantsthereof and preferably a combination of T1Rs or fragments or variantsthereof.

It is still another object of the invention to provide T1R fusionproteins or polypeptides which include at least a fragment of at leastone of such T1Rs.

It is another object of the invention to provide an isolated nucleicacid molecule encoding a T1R polypeptide comprising a nucleic acidsequence that is at least 50%, preferably 75%, 85%, 90%, 95%, 96%, 97%,98%, or 99% identical to a nucleic acid sequence having one of the hT1Rnucleic acid sequences identified infra, and conservatively modifiedvariants thereof.

It is a further object of the invention to provide an isolated nucleicacid molecule comprising a nucleic acid sequence that encodes apolypeptide having an amino acid sequence at least 35 to 50%, andpreferably 60%, 75%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical toan amino acid sequence selected from the group of one of the TIR aminoacid sequences identified infra and conservatively modified variantsthereof, wherein the fragment is at least 20, preferably 40, 60, 80,100, 150, 200, or 250 amino acids in length. Optionally, the fragmentcan be an antigenic fragment that binds to an anti-T1R antibody.

It is still a further object of the invention to provide an isolatedpolypeptide comprising a variant of said fragment, wherein there is avariation in at most 10, preferably 5, 4, 3, 2, or 1 amino acidresidues.

It is another object of the invention to provide T1R1/T1R3 combinationswherein T1R1 and/or T1R3 is a variant or fragment, and T1R2/T1R3combinations wherein T1R2 and/or T1R3 is a variant or fragment.

It is still another object of the invention to provide agonists orantagonists of such T1Rs, or fragments or variants thereof.

It is still another object of the invention to provide a PDZdomain-interacting peptide (herein referred to as PDZIP) which canfacilitate surface expression of integral plasma membrane proteins,specifically GPCRs such as the T1Rs. It is also an object of theinvention to provide vectors including PDZIP, host cells expressing suchvectors, and methods of using PDZIP to facilitate surface expression.

It is a preferred object of the invention to provide assays, especiallyhigh-throughput assays, for identifying taste-modulating compounds,particularly sweet taste and umami taste modulating compounds.Preferably, such assays will utilize a combination of T1Rs, or fragmentsor variants thereof, or genes encoding such T1Rs, or fragments orvariants thereof, which are disclosed herein. Most preferably suchcombinations will comprise hT1R1/hT1R3 and hT1R2/hT1R3.

It is an especially preferred embodiment of the invention to identifycompounds that modulate the T1R1/T1R3 or T1R2/T1R3 taste receptors,e.g., which enhance the ability of these receptors to respond to tastestimuli. For example, as described infra, it has been discovered that5′-IMP or 5′-GMP enhances the responsiveness of the umami (T1R1/T1R3) toL-glutamate. These modulatory compounds may enhance the activity ofdifferent sweet or umami taste stimuli, and provide for enhanced tastesand/or for the same taste to be elicited at reduced concentration of theparticular sweet or umami taste eliciting compound the activity of whichis enhanced by a taste modulator identified using the subject assays.

It is still a further object of the invention to provide preferredassays for evaluating one or more compounds for a taste comprising: astep of contacting said one or more compounds with at least one of thedisclosed T1Rs, fragments or variants thereof, preferably combinationsof human T1Rs.

It is a more specific object of the invention to provide a method ofscreening one or more compounds for their ability to enhance, mimic,block and/or modulate sweet taste perception, in a mammal, preferablyhuman, comprising a step of contacting one or more compounds with acombination of hT1R2 and hT1R3 or a complex comprising a fragment,chimera, or variant of hT1R2 and/or hT1R3.

It is another specific object of the invention to provide a method ofscreening one or more compounds for their ability to enhance, mimic,block and/or modulate taste perception, especially umami tasteperception in a mammal, preferably human, comprising a step ofcontacting said one or more compounds with a combination of hT1R1 andhT1R3, or a complex comprising a fragment, chimera, or variant of hT1R1and hT1R3.

It is another specific object of the invention to produce cells thatco-express hT1R2 and hT1R3, or a fragment, variant or chimera thereof,for use in identifying compounds that enhance, mimic, block and/ormodulate taste perception, especially sweet taste perception.

It is another specific object of the invention to produce cells thatco-express hT1R1 and hT1R3 or a fragment, variant or chimera thereof foruse in assays for identifying compounds that enhance, mimic, blockand/or modulate taste perception, especially umami taste perception.

It is another object of the invention to produce non-human animals thathave been genetically modified to express or not express one or moreT1Rs.

It is yet another object of the invention to utilize a compoundidentified using an assay that utilizes T1Rs, or a combination thereof,as flavor ingredients in food and beverage compositions. In particular,it is an object of the invention to utilize a compound that interactswith hT1R2 and/or hT1R3 as a sweet blocker, enhancer, modulator, ormimic, and a compound that interacts with hT1R1 and/or hT1R3 as a umamiblocker, enhancer, modulator, or mimic in food and beveragecompositions.

It is another object of the invention to use T1Rs, in particularnon-human T1Rs, to identify compounds that modulate the taste of animalfeed formulations for use in, e.g., fish aquaculture.

It is a preferred object of the invention to provide eukaryotic,preferably mammalian or insect cell lines that stably co-expresshT1R1/hT1R3 or hT1R2/hT1R3, preferably HEK-293 cell lines, which alsoexpress a G protein, e.g., Gα15 or another G protein that when expressedin association with T1R2/T1R3 or T1R1/T1R3 produces a functional tastereceptor.

It is another preferred object of the invention to provide eukaryoticcell lines, preferably mammalian or insect cells, that stably expressT1R1/T1R3 or T1R2/T1R3, preferably hT1R1/hT1R3 or hT1R2/hT1R3. In apreferred embodiment such cells will comprise HEK-293 cells that stablyexpress Gα15 or another G protein that associates with T1R1/T1R3 orT1R2/T1R3 to produce a functional umami or sweet taste receptor.

It is also an object of the invention to provide assays, preferably highthroughput assays using HEK-293 or other cell lines that stably ortransiently express T1R1/T1R3 or T1R2/T1R3, under constitutive orinducible conditions to identify compounds that modulate umami or sweettaste.

It is another specific object of the invention to identify compoundsthat enhance, mimic, block and/or modulate the T1R1/T1R3 umami tastereceptor based on their ability to affect the binding of lactisole (asweet taste inhibitor) or a structurally related compound to theT1R1/T1R3 (umami) taste receptor.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1B contains a sequence alignment of human (SEQ ID NOS: 5-7) andrat T1Rs (SEQ ID NOS: 16-17 and 4), human calcium-sensing receptor (SEQID NO: 19) and rat metabotropic glutamate receptor (SEQ ID NO: 18).

FIG. 2 contains RT-PCR amplification experimental results which showthat hT1R2 and hT1R3 are expressed in taste tissue. hT1R2 and hT1R3 areexpressed in human tongue epithelium. cDNA-specific amplificationproducts can be amplified from cDNA prepared from resected circumvallatepapillae.

FIGS. 3A-3C. Human T1R2/T1R3 functions as a sweet taste receptor. FIGS.3A-3B contain functional data (intracellular calcium responses) elicitedby different sweet taste stimuli in HEK cells stably expressing G_(α15)that are transiently transfected with human T1R2, T1R3 and T1R2/T1R3 atvarious concentrations of sweet taste stimuli (FIG. 3A); human T1R2/T1R3dose responses for several sweet taste stimuli (FIG. 3B); humanT1R2/T1R3 responses to sucrose in the presence of gurmarin, andendogenous β2-adrenergic receptor responses to isoproterenol in thepresence of gurmarin. FIG. 3C contains the normalized response todifferent sweeteners.

FIG. 4. T1R2 may control T1R2/T1R3 ligand specificity. FIG. 4 containsintracellular calcium responses in HEK cells stably expressing Gα15,transiently transfected with hT1R2/hT1R3, rT1R2/rT1R3, hT1R2/rT1R3 andrT1R2/hT1R3 in response to 350 mM sucrose, 25 mM tryptophan, 15 mMaspartame, and 0.05% monellin.

FIG. 5 contains the results of a fluorescence plate reactor based assaywherein HEK cells stably expressing Gα15 were transiently transfectedwith hT1R2 and hT1R3 or hT1R3 alone and contacted with the calcium dyeFluo-4 and a sweet taste stimulus (12.5 mM cyclamate).

FIG. 6 contains normalized dose-response curves which show that hT1R2and hT1R3 function in combination as the human sweet receptor based ontheir dose-specific interaction with various sweet stimuli (trp,cyclamate, sucrose, neotame, asparame, saccharin and Acek).

FIG. 7. Key ligand-binding residues of mGluR1 are conserved in T1R1.FIG. 7 contains structural information relating to mGluR1 and T1R1showing the key ligand binding residues are observed in these molecules.

FIGS. 8A-8C. Human T1R1/T1R3 functions as a umami taste receptor. FIGS.8A-8C contain functional data showing HEK cells which stably expressGa15 that are transiently transfected with T1R1/T1R3 respond toglutamate in an intracellular calcium-based assay. FIG. 8A shows thatintracellular calcium increases in response to increasing glutamateconcentration; FIG. 8B shows intracellular calcium responds to IMP (2mM), glutamate (0.5 mM) and 0.2 mM IMP; and FIG. 8C shows humanT1R1/T1R3 responses for glutamate in the presence and absence of 0.2 mMIMP.

FIGS. 9A-9B respectively, contain the results of an immunofluorescencestaining assay using Myc-tagged hT1R2 and a FACS experiment showing thatthe incorporation of the PDZIP peptide (SEQ ID No: 1) enhanced theexpression of a T1R (hT1R2) on the plasma membrane.

FIGS. 10A-10L contain calcium imaging data demonstrating thath1TR2/hT1R3 respond to different sweet stimuli.

FIG. 11 shows the responses of cell lines which stably expresshT1R1/hT1R3 by automated fluorescence imaging to umami taste stimuli.

FIG. 12 shows the responses of a cell line which stably expresseshT1R2/hT1R3 by automated fluorescence imaging to sweet taste stimuli.

FIG. 13 shows dose-response curves determined using automatedfluorescence imaging for a cell line that inducibly expresses the humanT1R1/T1R3 taste receptor for L-glutamate in the presence and absence of0.2 mM IMP.

FIG. 14 shows the response of a cell line that inducibly expresses thehuman T1R1/T1R3 taste receptor (I-17 clone) to a panel of L-amino acids.In FIG. 14 different L-amino acids at 10 mM were tested in the presenceand absence of 1 mM IMP.

FIG. 15 shows the response of a cell line that inducibly expresses thehuman T1R1/T1R3 taste receptor (I-17 clone) to a panel of L-amino acids.In FIG. 15 dose-responses for active amino acids were determined in thepresence of 0.2 mM IMP.

FIGS. 16A-16B show that lactisole inhibits the receptor activities ofhuman T1R2/T1R3 and human T1R1/T1R3. FIG. 16A shows responses ofHEK-Gα15 cells transiently transfected with T1R1/T1R3 (circles) to 10 mML-glutamate and HEK-Gα15 cells transiently transfected with T1R2/T1R3(squares) to 150 mM sucrose in the presence of variable concentrationsof lactisole. FIG. 16B shows fold increases in taste detectionthresholds in the presence of 1 and 2 mM lactisole for the sweet tastestimuli sucrose and D-tryptophan, the umami taste stimuli L-glutamate(MSG) and L-glutamate plus 0.2 mM IMP, and sodium chloride. Detectionthresholds were determined following the method of Schiffman et al.

DETAILED DESCRIPTION OF THE INVENTION

The invention thus provides functional taste receptors, preferably humantaste receptors, that are produced by co-expression of a combination ofdifferent T1Rs, preferably T1R1/T1R3 or T1R2/T1R3, and the correspondingisolated nucleic acid sequences or fragments, chimeras, or variantsthereof that upon co-expression result in a functional taste receptor,i.e., a sweet taste receptor (T1R2/T1R3) or umami taste receptor(T1R1/T1R3).

As has been reported in the literature, Members of the T1R family oftaste-cell-specific GPCRs known and are identified in Hoon et al., Cell,96:541-551 (1999), WO 00/06592, WO 00/06593, and U.S. Ser. No.09/799,629, all of which are incorporated herein by reference in theirentireties.

More particularly, the invention relates to the co-expression ofdifferent taste-cell specific GPCRs. These nucleic acids and thereceptors that they encode are referred to as members of the “T1R”family of taste-cell-specific GPCRs. In particular embodiments of theinvention, the T1R family members that are co-expressed will includerT1R1, rT1R2, rT1R3, mT1R1, mT1R2, mT1R3, hT1R1, hT1R2 and hT1R3. Whilenot wishing to be bound by theory, it is believed that thesetaste-cell-specific GPCRs are components of the taste transductionpathway, and are involved in the taste detection of sweet and umamitaste stimuli and/or other taste stimuli representing other tastemodalities.

It is established herein that T1R family members act in combination withother T1R family members to function as sweet and umami taste receptors.As disclosed in further detail infra in the experimental examples, ithas been demonstrated that heterologous cells which co-express hT1R2 andhT1R3 are selectively activated by sweet taste stimuli in a manner thatmirrors human sweet taste. For example, HEK-293-Gα15 cells thatco-express hT1R2 and hT1R3 specifically respond to cyclamate, sucrose,aspartame, and saccharin, and the dose responses for these compoundscorrelate with the psychophysical taste detection thresholds. Therefore,cells that co-express hT1R2 and hT1R3 can be used in screens, preferablyhigh throughput screens, to identify compounds that mimic, modulate,block, and/or enhance sweet taste sensation.

Also, as supported by data in the experimental examples, it has beenshown that cells which co-express hT1R1 and hT1R3 are selectivelyactivated by glutamate (monosodium glutamate) and 5′-ribonucleotides ina manner that mirrors human umami taste. For example, HEK-293-Gα15 cellsthat co-express hT1R1 and hT1R3 specifically respond to glutamate andthe dose response for this umami-tasting compound correlates with itspsychophysical taste detection threshold. Moreover, 5′-ribonucleotidessuch as IMP enhance the glutamate response of the T1R1/T1R3 receptor, asynergism characteristic of umami taste. Therefore, cells thatco-express hT1R1 and hT1R3 can be used in screens, preferably highthroughput screens to identify compounds that mimic, modulate, block,and/or enhance umami taste sensation.

Further, as shown by experimental data in the examples it has been shownthat cells which stably and inducibly co-express T1R1/T1R3 selectivelyrespond to the umami taste stimuli L-glutamate and L-aspartate and onlyweakly respond to other L-amino acids, and at much higherconcentrations, providing further evidence that the T1R1/T1R3 receptorcan be used in assays to identify compounds that modulate (enhance orblock) umami taste stimuli.

Also, as supported by experimental data in the examples, it has beenshown that cell lines which co-express T1R1/T1R3 or T1R2/T1R3respectively respond to umami or sweet taste stimuli and a quantitativedose-responsive manner which further supports a conclusion that theT1R1/T1R3 and T1R2/T1R3 receptor can be used to identify receptoragonists and antagonists, e.g., MSG substitutes, umami blockers, novelartificial and natural sweeteners, and sweet blockers.

Also, as supported by data in experimental examples, it has been shownthat the sweet taste blocker lactisole inhibits both the T1R2/T1R3 sweetreceptor and the T1R1/T1R3 umami taste receptor. This suggests thatassays which screen for compounds which affect the binding of lactisoleto T1R2/T1R3 or T1R1/T1R3 may be used to identify compounds thatenhance, mimic, modulate or block sweet or umami taste. The fact thatlactisole inhibits both the T1R1/T1R3 and T1R2/T1R3 receptors suggeststhat these receptors may share a common subunit which is bound bylactisole and potentially other taste modulators. Therefore, thissuggests that some compounds which enhance, mimic, modulate or blocksweet taste may have a similar effect on umami taste or vice versa.

Further, as supported by data in experimental examples, it has beendemonstrated that cell lines which stably co-express T1Rs, i.e.T1R1/T1R3 or T1R2/T1R3, when assayed by automated fluorescence imagingvery effectively respond to various sweet and umami taste stimuli, i.e.at magnitudes substantially greater than transiently transfected cells.Thus, these cell lines are especially well suited for use in highthroughput screening assays for identifying compounds that modulate,block, mimic or enhance sweet or umami taste. However, the inventionalso encompasses assays that utilize cells that transiently express aT1R or combination thereof.

Moreover, while the application contains data demonstrating that someT1Rs act in combination, particularly T1R1/T1R3 and T1R2/T1R3, and thatsuch receptor combinations may be used in assays, preferably highthroughput assays, it should be noted that the subject invention alsoencompasses assays that utilize T1R1, T1R2 and T1R3 alone or incombination with other proteins, e.g., other GPCRs.

With respect thereto, it is speculated that the sweet receptors may becomprised only of T1R2 and/or that the umami receptor may be comprisedonly of T1R1, with the T1R3 receptor perhaps functioning to facilitatethe surface expression of T1R2 or T1R1.

Alternatively, it is hypothesized that the sweet receptor and the umamireceptor may be composed only of T1R3, which may be differentiallyprocessed under the control of T1R1 and/or T1R2. This type of receptorexpression would be akin to the RAMP-dependent processing of thecalcitonin-related receptor.

Compounds identified with T1R assays can be used to modulate the tasteof foods and beverages. Suitable assays described in further detailinfra include by way of example whole-cell assays and biochemicalassays, including direct-binding assays using one of a combination ofdifferent T1R receptors, chimeras or fragments thereof, especiallyfragments containing N-terminal ligand-binding domains. Examples ofassays appropriate for use in the invention are described in greaterdetail infra and are known in the GPCR field.

Assays can be designed that quantitate the binding of differentcompounds or mixtures of compounds to T1R taste receptors or T1R tastereceptor combinations or T1R receptors expressed in combination withother heterologous (non-T1R) proteins, e.g. other GPCRs, or thatquantitate the activation of cells that express T1R taste receptors.This can be effected by stably or transiently expressing taste receptorsin heterologous cells such as HEK-293, CHO and COS cells.

The assays will preferably use cells that also express (preferablystably) a G protein such as Gα15 or Gα16 or other promiscuous G proteinsor G protein variants, or an endogenous G protein. In addition, G_(β)and G_(γ) proteins may also be expressed therein.

The effect of a compound on sweet or umami taste using cells orcompositions that express or contain the above-identified receptors orreceptor combinations may be determined by various means including theuse of calcium-sensitive dyes, voltage-sensitive dyes, cAMP assays,direct binding assays using fluorescently labeled ligands or radioactiveligands such as ³H-glutamate, or transcriptional assays (using asuitable reporter such as luciferase or beta-lactamase).

Assays that may be utilized with one or more T1Rs according to theinvention include by way of example, assays that utilize a geneticselection for living cells; assays that utilize whole cells or membranefragments or purified T1R proteins; assays that utilize secondmessengers such as cAMP and IP3, assays that detect the translocation ofarrestin to the cell surface, assays that detect the loss of receptorexpression on the cell surface (internalization) by tested ligands,direct ligand-binding assays, competitive-binding assays withinhibitors, assays using in vitro translated protein, assays that detectconformational changes upon the binding of a ligand (e.g., as evidencedby proteolysis, fluorescence, or NMR), behavioral assays that utilizetransgenic non-human animals that express a T1R or T1R combination, suchas flies, worms, or mice, assays that utilize cells infected withrecombinant viruses that contain T1R genes.

Also within the scope of the invention are structure-based analyseswherein the X-ray crystal structure of a T1R or T1R fragment (orcombination of T1Rs, or a combination of a T1R with another protein) isdetermined and utilized to predict by molecular modeling techniquescompounds that will bind to and/or enhance, mimic, block or modulate theparticular T1R receptor or receptor combination. More particularly, theinvention embraces the determination of the crystal structure ofT1R1/T1R3 (preferably hT1R1/hT1R3) and/or T1R2/T1R3 (preferablyhT1R2/hT1R3) and the use of such crystal structures in structure-baseddesign methods to identify molecules that modulate T1R receptoractivity.

The invention especially includes biochemical assays conducted usingcells, e.g., mammalian, yeast, insect or other heterologous cells thatexpress one or more full length T1R receptors or fragments, preferablyN-terminal domains of T1R1, T1R2 and/or T1R3. The effect of a compoundin such assays can be determined using competitive binding assays, e.g.,using radioactive glutamate or IMP, fluorescence (e.g., fluorescencepolarization, FRET), or GTPγ ³⁵S binding assays. As noted, in apreferred embodiment, such assays will utilize cell lines that stablyco-express T1R1/T1R3 or T1R2/T1R3 and a suitable G protein, such asG_(α15). Other appropriate G proteins include the chimeric and variant Gproteins disclosed in U.S. application Ser. Nos. 09/984,292 and60/243,770, incorporated by reference in their entirety herein.

Still further, altered receptors can be constructed and expressed havingimproved properties, e.g., enhanced surface expression or G-proteincoupling. These T1R variants can be incorporated into cell-based andbiochemical assays.

It is envisioned that the present discoveries relating to human T1Rswill extend to other species, e.g., rodents, pigs, monkeys, dogs andcats, and perhaps even non-mammals such as fish. In this regard, severalfish T1R fragments are identified infra in Example 1. Therefore, thesubject invention has application in screening for compounds for use inanimal feed formulations.

The invention further includes that utilize different allelic variantsof various T1Rs and combinations thereof, thereby enabling theidentification of compounds that elicit specific taste sensation inindividuals that express those allelic variants or compounds that elicitspecific taste sensations in all individuals. Such compounds can be usedto make foods more generally palatable.

T1R encoding nucleic acids also provide valuable probes for theidentification of taste cells, as the nucleic acids are specificallyexpressed in taste cells. For example, probes for T1R polypeptides andproteins can be used to identify taste cells present in foliate,circumvallate, and fungiform papillae, as well as taste cells present inthe geschmackstreifen, oral cavity, gastrointestinal epithelium, andepiglottis. In particular, methods of detecting T1Rs can be used toidentify taste cells sensitive to sweet and/or umami taste stimuli orother taste stimuli representing other taste modalities. For example,cells stably or transiently expressing T1R2 and/or T1R3 would bepredicted from the work herein to be responsive to sweet taste stimuli.Similarly, cells expressing T1R1 and/or T1R3 would be predicted to beresponsive to umami taste stimuli. The nucleic acids encoding the T1Rproteins and polypeptides of the invention can be isolated from avariety of sources, genetically engineered, amplified, synthesized,and/or expressed recombinantly according to the methods disclosed in WO00/035374, which is herein incorporated by reference in its entirety. Alisting of T1Rs that may be expressed according to the invention areprovided in the Examples. However, it should be emphasized that theinvention embraces the expression and use of other specific T1Rs orfragments, variants, or chimeras constructed based on such T1Rsequences, and particularly T1Rs of other species.

As disclosed, an important aspect of the invention is the plurality ofmethods of screening for modulators, e.g., activators, inhibitors,stimulators, enhancers, agonists, and antagonists, of thesetaste-cell-specific GPCRs. Such modulators of taste transduction areuseful for the modulation of taste signaling pathways. These methods ofscreening can be used to identify high affinity agonists and antagonistsof taste cell activity. These modulatory compounds can then be used inthe food industry to customize taste, e.g., to modulate the sweet and/orumami tastes of foods.

This invention rectifies the previous lack of understanding relating tosweet and umami taste as it identifies specific T1Rs and T1R receptorcombinations that mediate sweet and umami taste sensation. Therefore, ingeneral, this application relates to the inventors' discoveries relatingto the T1R class of taste-specific G-protein-coupled receptors and theirspecific function in taste perception and the relationship of thesediscoveries to a better understanding of the molecular basis of taste.

The molecular basis of sweet taste and umami taste—the savor ofmonosodium glutamate—is enigmatic. Recently, a three-member class oftaste-specific G-protein-coupled receptors, termed T1Rs, was identified.Overlapping T1R expression patterns and the demonstration that thestructurally related GABA_(B) receptor is heterodimeric suggest that theT1Rs function as heterodimeric taste receptors. In the examples infra,the present inventors describe the functional co-expression of humanT1R1, T1R2, and T1R3 in heterologous cells; cells co-expressing T1R1 andT1R3 are activated by umami taste stimuli; cells co-expressing T1R2 andT1R3 are activated by sweet taste stimuli. T1R1/T1R3 and T1R2/T1R3activity correlated with psychophysical detection thresholds. Inaddition, the 5′-ribonucleotide IMP was found to enhance the T1R1/T1R3response to glutamate, a synergism characteristic of umami taste. Thesefindings demonstrate that specific T1Rs and particularly differentcombinations of the T1Rs function as sweet and umami taste receptors.

Human perception of bitter, sweet, and umami is thought to be mediatedby G-protein-coupled receptors (Lindemann, B., Physiol. Res. 76:718-66(1996)). Recently, evaluation of the human genome revealed the T2R classof bitter taste receptors (Adler et al., Cell 100:613-702 (2000);Chandrasgekar et al., Cell 100:703-11 (2000); Matsunami et al., Nature404: 601-604 (2000)) but the receptors for sweet and umami taste havenot been identified. Recently, another class of candidate tastereceptors, the T1Rs, was identified. The T1Rs were first identified bylarge-scale sequencing of a subtracted cDNA library derived from rattaste tissue, which identified T1R1, and subsequently by T1R1-baseddegenerate PCR, which led to the identification of T1R2 (Hoon et al.,Cell 96:541-551 (1999)). Recently, the present inventors and othersidentified a third and possibly final member of the T1R family, T1R3, inthe human genome databank (Kitagawa et al., Biochem Biophys. Res Commun.283(1): 236-42 (2001); Max et al., Nat. Genet. 28(1): 58-63 (2001);Sainz et al., J. Neurochem. 77(3): 896-903 (2001); Montmayeur et al.,Nat. Neurosci. 4, 492-8. (2001)). Tellingly, mouse T1R3 maps to agenomic interval containing Sac, a locus that influences sweet taste inthe mouse (Fuller et al., J. Hered. 65:33-6 (1974); Li et al., Mamm.Genome 12:13-16 (2001)). Therefore, T1R3 was predicted to function as asweet taste receptor. Recent high-resolution genetic mapping studieshave strengthened the connection between mouse T1R3 and Sac (Fuller T.C., J. Hered. 65(1): 33-36 (1974); Li et al., Mammal. Genome 12(1):13-16 (2001)).

Interestingly, all C-family receptors that have been functionallyexpressed thus far—metabotropic glutamate receptors, the GABA_(B)receptor, the calcium-sensing receptor (Conigrave, A. D., Quinn, S. J. &Brown, E. M., Proc Natl Acad Sci USA 97, 4814-9. (2000)), and a fisholfactory receptor (Speca, D. J. et al., Neuron 23, 487-98. (1999))—havebeen shown to be activated by amino acids. This common feature raisesthe possibility that the T1Rs recognize amino acids, and that the T1Rsmay be involved in the detection of glutamate in addition tosweet-tasting amino acids. Alternatively, a transcriptional variant ofthe mGluR4 metabotropic glutamate receptor has been proposed to be theumami taste receptor because of its selective expression in rat tastetissue, and the similarity of the receptor-activation threshold to theglutamate psychophysical detection threshold (Chaudhari et al., Nat.Neurosci. 3:113-119 (2000)). This hypothesis is difficult to reconcilewith the exceedingly low expression level of the mGluR4 variant in tastetissue, and the more or less unaltered glutamate taste of mGluR4knockout mice (Chaudhari and Roper, Ann. N.Y. Acad. Sci. 855:398-406(1998)). Furthermore, the taste variant is structurally implausible,lacking not only the majority of the residues that form theglutamate-binding pocket of the wild-type receptor, but alsoapproximately half of the globular N-terminal glutamate-binding domain(Kunishima et al., Nature 407:971-7 (2000)).

Comparative analysis of T1R expression patterns in rodents hasdemonstrated that T1R2 and possibly T1R1 are each coexpressed with T1R3(Hoon et al., Cell 96:541-51 (1999); Kitagawa et al., Biochem Biophy.Res. Commun. 283:236-242 (2001); Max et al., Nat. Genet. 28:58-63(2001); Montmayeur et al., Nat. Neurosci 4:492-8 (2001); Sainz et al.,J. Neurochem 77:896-903 (2001)). Furthermore, dimerization is emergingas a common theme of C-family receptors: the metabotropic glutamate andcalcium-sensing receptor are homodimers (Romomano et al., J. Biol. Chem.271:28612-6 (1996); Okamoto et al., J. Biol. Chem. 273: 13089-96 (1998);Han et al., J. Biol. Chem. 274:100008-13 (1999); Bai et al., J. Biol.Chem. 273:23605-10 (1998)), and the structurally related GABA_(B)receptor is heterodimeric (Jones et al., Nature 396:674-9 (1998);Kaupmann et al., Nature 396:683-687 (1998); White et al., Nature 396:679-682 (1998); Kuner et al., Science 283:74-77 (1999)). The presentinventors have demonstrated by functional coexpression of T1Rs inheterologous cells that human T1R2 functions in combination with humanT1R3 as a sweet taste receptor and that human T1R1 functions incombination with human T1R3 as an umami taste receptor.

The discoveries discussed herein are especially significant, aspreviously the development of improved artificial sweeteners has beenhampered by the lack of assays for sweet taste. Indeed, the fivecommonly used commercial artificial sweeteners, all of which activatehT1R2/hT1R3, were discovered serendipitously. Similarly, other thansensory testing, a laborious process, there is no assay for identifyingcompounds that modulate umami taste. These problems are now alleviatedbecause, as established by experimental results discussed infra, thehuman sweet and umami receptors have been identified, and assays forthese receptors have been developed, particularly assays that use cellsthat stably express a functional T1R taste receptor, i.e. the sweet orumami taste receptor.

Based thereon the invention provides assays for detecting andcharacterizing taste-modulating compounds, wherein T1R family membersact, as they do in the taste bud, as reporter molecules for the effecton sweet and umami taste of taste-modulating compounds. Particularlyprovided and within the scope of the invention are assays foridentifying compounds that modulate, mimic, enhance and/or blockindividually, sweet and umami tastes. Methods for assaying the activityof GPCRs, and especially compounds that affect GPCR activity are wellknown and are applicable to the T1R family member of the presentinvention and functional combinations thereof. Suitable assays have beenidentified supra.

In particular, the subject GPCRs can be used in assays to, e.g., measurechanges in ligand binding, ion concentration, membrane potential,current flow, ion flux, transcription, receptor-ligand interactions,second messenger concentrations, in vitro and in vivo. In anotherembodiment, T1R family members may be recombinantly expressed in cells,and the modulation of taste transduction via GPCR activity may beassayed by measuring changes in Ca²⁺ levels and other intracellularmessages such as cAMP, cGMP, or IP₃.

In certain assays, a domain of a T1R polypeptide, e.g., anextracellular, transmembrane, or intracellular domain, is fused to aheterologous polypeptide, thereby forming a chimeric polypeptide, e.g.,a chimeric protein with GPCR activity. Particularly contemplated is theuse of fragments of T1R1, T1R2 or T1R3 containing the N-terminalligand-binding domain. Such proteins are useful, e.g., in assays toidentify ligands, agonists, antagonists, or other modulators of T1Rreceptors. For example, a T1R polypeptide can be expressed in aeukaryotic cell as a chimeric receptor with a heterologous, chaperonesequence that facilitates plasma membrane trafficking, or maturation andtargeting through the secretory pathway. The optional heterologoussequence may be a PDZ domain-interacting peptide, such as a C-terminalPDZIP fragment (SEQ ID NO 1). PDZIP is an ER export signal, which,according to the present invention, has been shown to facilitate surfaceexpression of heterologous proteins such as the T1R receptors describedherein. More particularly, in one aspect of the invention, PDZIP can beused to promote proper targeting of problematic membrane proteins suchas olfactory receptors, T2R taste receptors, and the T1R taste receptorsdescribed herein.

Such chimeric T1R receptors can be expressed in any eukaryotic cell,such as HEK-293 cells. Preferably, the cells contain a G protein,preferably a promiscuous G protein such as G_(α15) or G_(α16) or anothertype of promiscuous G protein capable of linking a wide range of GPCRsto an intracellular signaling pathway or to a signaling protein such asphospholipase C. Activation of such chimeric receptors in such cells canbe detected using any standard method, such as by detecting changes inintracellular calcium by detecting FURA-2 dependent fluorescence in thecell. If preferred host cells do not express an appropriate G protein,they may be transfected with a gene encoding a promiscuous G proteinsuch as those described in U.S. Application Ser. No. 60/243,770, U.S.application Ser. No. 09/984,297, filed Oct. 29, 2001, and U.S.application Ser. No. 09/989,497 filed Nov. 21, 2001 which are hereinincorporated by reference in its entirety.

Additional methods of assaying for modulators of taste transductioninclude in vitro ligand-binding assays using: T1R polypeptides, portionsthereof, i.e., the extracellular domain, transmembrane region, orcombinations thereof, or chimeric proteins comprising one or moredomains of a T1R family member; oocyte or tissue culture cellsexpressing T1R polypeptides, fragments, or fusion proteins;phosphorylation and dephosphorylation of T1R family members; G proteinbinding to GPCRs; ligand-binding assays; voltage, membrane potential andconductance changes; ion flux assays; changes in intracellular secondmessengers such as cGMP, cAMP and inositol triphosphate (IP3); andchanges in intracellular calcium levels.

Further, the invention provides methods of detecting T1R nucleic acidand protein expression, allowing investigation of taste transductionregulation and specific identification of taste receptor cells. T1Rfamily members also provide useful nucleic acid probes for paternity andforensic investigations. T1R genes are also useful as nucleic acidprobes for identifying taste receptor cells, such as foliate, fungiform,circumvallate, geschmackstreifen, and epiglottis taste receptor cells.T1R receptors can also be used to generate monoclonal and polyclonalantibodies useful for identifying taste receptor cells.

Functionally, the T1R polypeptides comprise a family of related seventransmembrane G protein-coupled receptors, which are believed to beinvolved in taste transduction and may interact with a G protein tomediate taste signal transduction (see, e.g., Fong, Cell Signal, 8:217(1996); Baldwin, Curr. Opin. Cell Biol., 6:180 (1994)). Structurally,the nucleotide sequences of T1R family members encode relatedpolypeptides comprising an extracellular domain, seven transmembranedomains, and a cytoplasmic domain. Related T1R family genes from otherspecies share at least about 50%, and optionally 60%, 70%, 80%, or 90%,nucleotide sequence identity over a region of at least about 50nucleotides in length, optionally 100, 200, 500, or more nucleotides inlength to the T1R nucleic acid sequences disclosed herein in theExamples, or conservatively modified variants thereof, or encodepolypeptides sharing at least about 35 to 50%, and optionally 60%, 70%,80%, or 90%, amino acid sequence identity over an amino acid region atleast about 25 amino acids in length, optionally 50 to 100 amino acidsin length to a T1R polypeptide sequence disclosed infra in the Examplesconservatively modified variants thereof.

Several consensus amino acid sequences or domains have also beenidentified that are characteristic of T1R family members. For example,T1R family members typically comprise a sequence having at least about50%, optionally 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95-99%, orhigher, identity to T1R consensus sequences 1 and 2 (SEQ ID NOs. 2 and3, respectively). These conserved domains thus can be used to identifymembers of the T1R family, by identity, specific hybridization oramplification, or specific binding by antibodies raised against adomain. T1R consensus sequences include by way of example the followingsequences:

T1R Family Consensus Sequence 1: (SEQ ID NO: 2)(TR)C(FL)(RQP)R(RT)(SPV)(VERKT)FL(AE)(WL)(RHG)ET1R Family Consensus Sequence 2: (SEQ ID NO: 3)(LQ)P(EGT)(NRC)YN(RE)A(RK)(CGF)(VLI)T(FL)(AS)(ML)

These consensus sequences are inclusive of those found in the T1Rpolypeptides described herein, but T1R family members from otherorganisms may be expected to comprise consensus sequences having about75% identity or more to the inclusive consensus sequences describedspecifically herein.

Specific regions of the T1R nucleotide and amino acid sequences may beused to identify polymorphic variants, interspecies homologs, andalleles of T1R family members. This identification can be made in vitro,e.g., under stringent hybridization conditions or PCR (e.g., usingprimers encoding the T1R consensus sequences identified above), or byusing the sequence information in a computer system for comparison withother nucleotide sequences. Different alleles of T1R genes within asingle species population will also be useful in determining whetherdifferences in allelic sequences control differences in taste perceptionbetween members of the population. Classical PCR-type amplification andcloning techniques are useful for isolating new T1Rs, for example, wheredegenerate primers are sufficient for detecting related genes acrossspecies.

Typically, identification of polymorphic variants and alleles of T1Rfamily members can be made by comparing an amino acid sequence of about25 amino acids or more, e.g., 50-100 amino acids. Amino acid identity ofapproximately at least 35 to 50%, and optionally 60%, 70%, 75%, 80%,85%, 90%, 95-99%, or above typically demonstrates that a protein is apolymorphic variant, interspecies homolog, or allele of a T1R familymember. Sequence comparison can be performed using any of the sequencecomparison algorithms discussed below. Antibodies that bind specificallyto T1R polypeptides or a conserved region thereof can also be used toidentify alleles, interspecies homologs, and polymorphic variants.

Polymorphic variants, interspecies homologs, and alleles of T1R genescan be confirmed by examining taste-cell-specific expression of theputative T1R gene or protein. Typically, T1R polypeptides having anamino acid sequence disclosed herein can be used as a positive controlin comparison to the putative T1R polypeptide to demonstrate theidentification of a polymorphic variant or allele of the T1R familymember. The polymorphic variants, alleles, and interspecies homologs areexpected to retain the seven transmembrane structure of a Gprotein-coupled receptor. For further detail, see WO 00/06592, whichdiscloses related T1R family members, GPCR-B3s, the contents of whichare herein incorporated by reference in a manner consistent with thisdisclosure. GPCR-B3 receptors are referred to herein as rT1R1 and mT1R1.Additionally, see WO 00/06593, which also discloses related T1R familymembers, GPCR-B4s, the contents of which are herein incorporated byreference in a manner consistent with this disclosure. GPCR-B4 receptorsare referred to herein as rT1R2 and mT1R2. As discussed previously, theinvention also includes structure-based assays that utilize the x-raycrystalline structure of a T1R or T1R combination, e.g., hT1R2/hT1R3 orhT1R1/hT1R3, to identify molecules that modulate T1R receptor activity,and thereby modulate sweet and/or umami taste.

The present invention also provides assays, preferably high throughputassays, to identify molecules that enhance, mimic, block and/or modulateT1R receptors. In some assays, a particular domain of a T1R familymember is used in combination with a particular domain of another T1Rfamily member, e.g., an extracellular, transmembrane, or intracellulardomain or region. In other embodiments, an extracellular domain,transmembrane region or combination thereof may be bound to a solidsubstrate, and used, e.g., to isolate ligands, agonists, antagonists, orany other molecules that can bind to and/or modulate the activity of aT1R polypeptide.

Various conservative mutations and substitutions are envisioned to bewithin the scope of the invention. For instance, it is within the levelof skill in the art to perform amino acid substitutions using knownprotocols of recombinant gene technology including PCR, gene cloning,site-directed mutagenesis of cDNA, transfection of host cells, andin-vitro transcription. The variants could then be screened foractivity.

Definitions

As used herein, the following terms have the meanings ascribed to themunless specified otherwise.

“Taste cells” include neuroepithelial cells that are organized intogroups to form taste buds of the tongue, e.g., foliate, fungiform, andcircumvallate cells (see, e.g., Roper et al., Ann. Rev. Neurosci.12:329-353 (1989)). Taste cells are also found in the palate and othertissues, such as the esophagus and the stomach.

“T1R” refers to one or more members of a family of G protein-coupledreceptors that are expressed in taste cells such as foliate, fungiform,and circumvallate cells, as well as cells of the palate, and esophagus(see, e.g., Hoon et al., Cell, 96:541-551 (1999), herein incorporated byreference in its entirety). Members of this family are also referred toas GPCR-B3 and TR1 in WO 00/06592 as well as GPCR-B4 and TR2 in WO00/06593. GPCR-B3 is also herein referred to as rT1R1, and GPCR-B4 isreferred to as rT1R2. Taste receptor cells can also be identified on thebasis of morphology (see, e.g., Roper, supra), or by the expression ofproteins specifically expressed in taste cells. T1R family members mayhave the ability to act as receptors for sweet taste transduction, or todistinguish between various other taste modalities. Representative T1Rsequences, including hT1R1, hT1R2 and hT1R3 are identified infra in theexamples.

“T1R” nucleic acids encode a family of GPCRs with seven transmembraneregions that have “G protein-coupled receptor activity,” e.g., they maybind to G proteins in response to extracellular stimuli and promoteproduction of second messengers such as IP3, cAMP, cGMP, and Ca²⁺ viastimulation of enzymes such as phospholipase C and adenylate cyclase(for a description of the structure and function of GPCRs, see, e.g.,Fong, supra, and Baldwin, supra). A single taste cell may contain manydistinct T1R polypeptides.

The term “T1R” family therefore refers to polymorphic variants, alleles,mutants, and interspecies homologs that: (1) have at least about 35 to50% amino acid sequence identity, optionally about 60, 75, 80, 85, 90,95, 96, 97, 98, or 99% amino acid sequence identity to a T1Rpolypeptide, preferably those identified in Example 1, over a window ofabout 25 amino acids, optionally 50-100 amino acids; (2) specificallybind to antibodies raised against an immunogen comprising an amino acidsequence preferably selected from the group consisting of the T1Rpolypeptide sequence disclosed in Example 1 and conservatively modifiedvariants thereof; (3) are encoded by a nucleic acid molecule whichspecifically hybridize (with a size of at least about 100, optionally atleast about 500-1000 nucleotides) under stringent hybridizationconditions to a sequence selected from the group consisting of the T1Rnucleic acid sequences contained in Example 1, and conservativelymodified variants thereof; or (4) comprise a sequence at least about 35to 50% identical to an amino acid sequence selected from the groupconsisting of the T1R amino acid sequence identified in Example 1.

Topologically, certain chemosensory GPCRs have an “N-terminal domain;”“extracellular domains;” “transmembrane domains” comprising seventransmembrane regions, and corresponding cytoplasmic, and extracellularloops; “cytoplasmic domains,” and a “C-terminal domain” (see, e.g., Hoonet al., Cell, 96:541-551 (1999); Buck & Axel, Cell, 65:175-187 (1991)).These domains can be structurally identified using methods known tothose of skill in the art, such as sequence analysis programs thatidentify hydrophobic and hydrophilic domains (see, e.g., Stryer,Biochemistry, (3rd ed. 1988); see also any of a number of Internet basedsequence analysis programs, such as those found atdot.imgen.bcm.tmc.edu). Such domains are useful for making chimericproteins and for in vitro assays of the invention, e.g., ligand bindingassays.

“Extracellular domains” therefore refers to the domains of T1Rpolypeptides that protrude from the cellular membrane and are exposed tothe extracellular face of the cell. Such domains generally include the“N terminal domain” that is exposed to the extracellular face of thecell, and optionally can include portions of the extracellular loops ofthe transmembrane domain that are exposed to the extracellular face ofthe cell, i.e., the loops between transmembrane regions 2 and 3, betweentransmembrane regions 4 and 5, and between transmembrane regions 6 and7.

The “N-terminal domain” region starts at the N-terminus and extends to aregion close to the start of the first transmembrane domain. Moreparticularly, in one embodiment of the invention, this domain starts atthe N-terminus and ends approximately at the conserved glutamic acid atamino acid position 563 plus or minus approximately 20 amino acids.These extracellular domains are useful for in vitro ligand-bindingassays, both soluble and solid phase. In addition, transmembraneregions, described below, can also bind ligand either in combinationwith the extracellular domain, and are therefore also useful for invitro ligand-binding assays.

“Transmembrane domain,” which comprises the seven “transmembraneregions,” refers to the domain of T1R polypeptides that lies within theplasma membrane, and may also include the corresponding cytoplasmic(intracellular) and extracellular loops. In one embodiment, this regioncorresponds to the domain of T1R family members which startsapproximately at the conserved glutamic acid residue at amino acidposition 563 plus or minus 20 amino acids and ends approximately at theconserved tyrosine amino acid residue at position 812 plus or minusapproximately 10 amino acids. The seven transmembrane regions andextracellular and cytoplasmic loops can be identified using standardmethods, as described in Kyte & Doolittle, J. Mol. Biol., 157:105-32(1982)), or in Stryer, supra.

“Cytoplasmic domains” refers to the domains of T1R polypeptides thatface the inside of the cell, e.g., the “C-terminal domain” and theintracellular loops of the transmembrane domain, e.g., the intracellularloop between transmembrane regions 1 and 2, the intracellular loopbetween transmembrane regions 3 and 4, and the intracellular loopbetween transmembrane regions 5 and 6. “C-terminal domain” refers to theregion that spans the end of the last transmembrane domain and theC-terminus of the protein, and which is normally located within thecytoplasm. In one embodiment, this region starts at the conservedtyrosine amino acid residue at position 812 plus or minus approximately10 amino acids and continues to the C-terminus of the polypeptide.

The term “ligand-binding region” or “ligand-binding domain” refers tosequences derived from a taste receptor, particularly a taste receptorthat substantially incorporates at least the extracellular domain of thereceptor. In one embodiment, the extracellular domain of theligand-binding region may include the N-terminal domain and, optionally,portions of the transmembrane domain, such as the extracellular loops ofthe transmembrane domain. The ligand-binding region may be capable ofbinding a ligand, and more particularly, a compound that enhances,mimics, blocks, and/or modulates taste, e.g., sweet or umami taste.

The phrase “heteromultimer” or “heteromultimeric complex” in the contextof the T1R receptors or polypeptides of the invention refers to afunctional association of at least one T1R receptor and anotherreceptor, typically another T1R receptor polypeptide (or, alternativelyanother non-T1R receptor polypeptide). For clarity, the functionalco-dependence of the T1Rs is described in this application as reflectingtheir possible function as heterodimeric taste receptor complexes.However, as discussed previously, functional co-dependence mayalternatively reflect an indirect interaction. For example, T1R3 mayfunction solely to facilitate surface expression of T1R1 and T1R2, whichmay act independently as taste receptors. Alternatively, a functionaltaste receptor may be comprised solely of T1R3, which is differentiallyprocessed under the control of T1R1 or T1R2, analogous to RAMP-dependentprocessing of the calcium-related receptor.

The phrase “functional effects” in the context of assays for testingcompounds that modulate T1R family member mediated taste transductionincludes the determination of any parameter that is indirectly ordirectly under the influence of the receptor, e.g., functional, physicaland chemical effects. It includes ligand binding, changes in ion flux,membrane potential, current flow, transcription, G protein binding, GPCRphosphorylation or dephosphorylation, conformation change-based assays,signal transduction, receptor-ligand interactions, second messengerconcentrations (e.g., cAMP, cGMP, IP3, or intracellular Ca²⁺), in vitro,in vivo, and ex vivo and also includes other physiologic effects suchincreases or decreases of neurotransmitter or hormone release.

By “determining the functional effect” in the context of assays is meantassays for a compound that increases or decreases a parameter that isindirectly or directly under the influence of a T1R family member, e.g.,functional, physical and chemical effects. Such functional effects canbe measured by any means known to those skilled in the art, e.g.,changes in spectroscopic characteristics (e.g., fluorescence,absorbency, refractive index), hydrodynamic (e.g., shape),chromatographic, or solubility properties, patch clamping,voltage-sensitive dyes, whole cell currents, radioisotope efflux,inducible markers, oocyte T1R gene expression; tissue culture cell T1Rexpression; transcriptional activation of T1R genes; ligand-bindingassays; voltage, membrane potential and conductance changes; ion fluxassays; changes in intracellular second messengers such as cAMP, cGMP,and inositol triphosphate (IP3); changes in intracellular calciumlevels; neurotransmitter release, conformational assays and the like.

“Inhibitors,” “activators,” and “modulators” of T1R genes or proteinsare used to refer to inhibitory, activating, or modulating moleculesidentified using in vitro and in vivo assays for taste transduction,e.g., ligands, agonists, antagonists, and their homologs and mimetics.

Inhibitors are compounds that, e.g., bind to, partially or totally blockstimulation, decrease, prevent, delay activation, inactivate,desensitize, or down regulate taste transduction, e.g., antagonists.Activators are compounds that, e.g., bind to, stimulate, increase, open,activate, facilitate, enhance activation, sensitize, or up regulatetaste transduction, e.g., agonists. Modulators include compounds that,e.g., alter the interaction of a receptor with: extracellular proteinsthat bind activators or inhibitor (e.g., ebnerin and other members ofthe hydrophobic carrier family); G proteins; kinases (e.g., homologs ofrhodopsin kinase and beta adrenergic receptor kinases that are involvedin deactivation and desensitization of a receptor); and arrestins, whichalso deactivate and desensitize receptors. Modulators can includegenetically modified versions of T1R family members, e.g., with alteredactivity, as well as naturally occurring and synthetic ligands,antagonists, agonists, small chemical molecules and the like. Suchassays for inhibitors and activators include, e.g., expressing T1Rfamily members in cells or cell membranes, applying putative modulatorcompounds, in the presence or absence of tastants, e.g., sweet tastants,and then determining the functional effects on taste transduction, asdescribed above. Samples or assays comprising T1R family members thatare treated with a potential activator, inhibitor, or modulator arecompared to control samples without the inhibitor, activator, ormodulator to examine the extent of modulation. Positive control samples(e.g. a sweet tastant without added modulators) are assigned a relativeT1R activity value of 100%.

Negative control samples (e.g. buffer without an added taste stimulus)are assigned a relative T1R activity value of 0%. Inhibition of a T1R isachieved when a mixture of the positive control sample and a modulatorresult in the T1R activity value relative to the positive control isabout 80%, optionally 50% or 25-0%. Activation of a T1R by a modulatoralone is achieved when the T1R activity value relative to the positivecontrol sample is 10%, 25%, 50%, 75%, optionally 100%, optionally 150%,optionally 200-500%, or 1000-3000% higher.

The terms “purified,” “substantially purified,” and “isolated” as usedherein refer to the state of being free of other, dissimilar compoundswith which the compound of the invention is normally associated in itsnatural state, so that the “purified,” “substantially purified,” and“isolated” subject comprises at least 0.5%, 1%, 5%, 10%, or 20%, andmost preferably at least 50% or 75% of the mass, by weight, of a givensample. In one preferred embodiment, these terms refer to the compoundof the invention comprising at least 95% of the mass, by weight, of agiven sample. As used herein, the terms “purified,” “substantiallypurified,” and “isolated,” when referring to a nucleic acid or protein,also refers to a state of purification or concentration different thanthat which occurs naturally in the mammalian, especially human body. Anydegree of purification or concentration greater than that which occursnaturally in the mammalian, especially human, body, including (1) thepurification from other associated structures or compounds or (2) theassociation with structures or compounds to which it is not normallyassociated in the mammalian, especially human, body, are within themeaning of “isolated.” The nucleic acid or protein or classes of nucleicacids or proteins, described herein, may be isolated, or otherwiseassociated with structures or compounds to which they are not normallyassociated in nature, according to a variety of methods and processesknown to those of skill in the art.

The term “nucleic acid” or “nucleic acid sequence” refers to adeoxy-ribonucleotide or ribonucleotide oligonucleotide in either single-or double-stranded form. The term encompasses nucleic acids, i.e.,oligonucleotides, containing known analogs of natural nucleotides. Theterm also encompasses nucleic-acid-like structures with syntheticbackbones (see e.g., Oligonucleotides and Analogues, a PracticalApproach, ed. F. Eckstein, Oxford Univ. Press (1991); AntisenseStrategies, Annals of the N.Y. Academy of Sciences, Vol. 600, Eds.Baserga et al. (NYAS 1992); Milligan J. Med. Chem. 36:1923-1937 (1993);Antisense Research and Applications (1993, CRC Press), WO 97/03211; WO96/39154; Mata, Toxicol. Appl. Pharmacol. 144:189-197 (1997);Strauss-Soukup, Biochemistry 36:8692-8698 (1997); Samstag, AntisenseNucleic Acid Drug Dev, 6:153-156 (1996)).

Unless otherwise indicated, a particular nucleic acid sequence alsoimplicitly encompasses conservatively modified variants thereof (e.g.,degenerate codon substitutions) and complementary sequences, as well asthe sequence explicitly indicated. Specifically, degenerate codonsubstitutions may be achieved by generating, e.g., sequences in whichthe third position of one or more selected codons is substituted withmixed-base and/or deoxyinosine residues (Batzer et al., Nucleic AcidRes., 19:5081 (1991); Ohtsuka et al., J. Biol. Chem., 260:2605-2608(1985); Rossolini et al., Mol. Cell. Probes, 8:91-98 (1994)). The termnucleic acid is used interchangeably with gene, cDNA, mRNA,oligonucleotide, and polynucleotide.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical mimetic of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers and non-naturally occurring amino acid polymer.

The term “plasma membrane translocation domain” or simply “translocationdomain” means a polypeptide domain that, when incorporated into apolypeptide coding sequence, can with greater efficiency “chaperone” or“translocate” the hybrid (“fusion”) protein to the cell plasma membranethan without the domain. For instance, a “translocation domain” may bederived from the amino terminus of the bovine rhodopsin receptorpolypeptide, a 7-transmembrane receptor. However, rhodopsin from anymammal may be used, as can other translocation facilitating sequences.Thus, the translocation domain is particularly efficient intranslocating 7-transmembrane fusion proteins to the plasma membrane,and a protein (e.g., a taste receptor polypeptide) comprising an aminoterminal translocating domain will be transported to the plasma membranemore efficiently than without the domain. However, if the N-terminaldomain of the polypeptide is active in binding, as with the T1Rreceptors of the present invention, the use of other translocationdomains may be preferred. For instance, a PDZ domain-interactingpeptide, as described herein, may be used.

The “translocation domain,” “ligand-binding domain”, and chimericreceptors compositions described herein also include “analogs,” or“conservative variants” and “mimetics” (“peptidomimetics”) withstructures and activity that substantially correspond to the exemplarysequences. Thus, the terms “conservative variant” or “analog” or“mimetic” refer to a polypeptide which has a modified amino acidsequence, such that the change(s) do not substantially alter thepolypeptide's (the conservative variant's) structure and/or activity, asdefined herein. These include conservatively modified variations of anamino acid sequence, i.e., amino acid substitutions, additions ordeletions of those residues that are not critical for protein activity,or substitution of amino acids with residues having similar properties(e.g., acidic, basic, positively or negatively charged, polar ornon-polar, etc.) such that the substitutions of even critical aminoacids does not substantially alter structure and/or activity.

More particularly, “conservatively modified variants” applies to bothamino acid and nucleic acid sequences. With respect to particularnucleic acid sequences, conservatively modified variants refers to thosenucleic acids which encode identical or essentially identical amino acidsequences, or where the nucleic acid does not encode an amino acidsequence, to essentially identical sequences. Because of the degeneracyof the genetic code, a large number of functionally identical nucleicacids encode any given protein.

For instance, the codons GCA, GCC, GCG and GCU all encode the amino acidalanine. Thus, at every position where an alanine is specified by acodon, the codon can be altered to any of the corresponding codonsdescribed without altering the encoded polypeptide.

Such nucleic acid variations are “silent variations,” which are onespecies of conservatively modified variations. Every nucleic acidsequence herein, which encodes a polypeptide, also describes everypossible silent variation of the nucleic acid. One of skill willrecognize that each codon in a nucleic acid (except AUG, which isordinarily the only codon for methionine, and TGG, which is ordinarilythe only codon for tryptophan) can be modified to yield a functionallyidentical molecule. Accordingly, each silent variation of a nucleicacid, which encodes a polypeptide, is implicit in each describedsequence.

Conservative substitution tables providing functionally similar aminoacids are well known in the art. For example, one exemplary guideline toselect conservative substitutions includes (original residue followed byexemplary substitution): ala/gly or ser; arg/lys; asn/gln or his;asp/glu; cys/ser; gln/asn; gly/asp; gly/ala or pro; his/asn or gln;ile/leu or val; leu/ile or val; lys/arg or gln or glu; met/leu or tyr orile; phe/met or leu or tyr; ser/thr; thr/ser; trp/tyr; tyr/trp or phe;val/ile or leu. An alternative exemplary guideline uses the followingsix groups, each containing amino acids that are conservativesubstitutions for one another: 1) Alanine (A), Serine (S), Threonine(T); 2) Aspartic acid (D), Glutamic acid (E); 3) Asparagine (N),Glutamine (Q); 4) Arginine (R), Lysine (I); 5) Isoleucine (I), Leucine(L), Methionine (M), Valine (V); and 6) Phenylalanine (F), Tyrosine (Y),Tryptophan (W); (see also, e.g., Creighton, Proteins, W.H. Freeman andCompany (1984); Schultz and Schimer, Principles of Protein Structure,Springer-Vrlag (1979)). One of skill in the art will appreciate that theabove-identified substitutions are not the only possible conservativesubstitutions. For example, for some purposes, one may regard allcharged amino acids as conservative substitutions for each other whetherthey are positive or negative. In addition, individual substitutions,deletions or additions that alter, add or delete a single amino acid ora small percentage of amino acids in an encoded sequence can also beconsidered “conservatively modified variations.”

The terms “mimetic” and “peptidomimetic” refer to a synthetic chemicalcompound that has substantially the same structural and/or functionalcharacteristics of the polypeptides, e.g., translocation domains,ligand-binding domains, or chimeric receptors of the invention. Themimetic can be either entirely composed of synthetic, non-naturalanalogs of amino acids, or may be a chimeric molecule of partly naturalpeptide amino acids and partly non-natural analogs of amino acids. Themimetic can also incorporate any amount of natural amino acidconservative substitutions as long as such substitutions also do notsubstantially alter the mimetic's structure and/or activity.

As with polypeptides of the invention which are conservative variants,routine experimentation will determine whether a mimetic is within thescope of the invention, i.e., that its structure and/or function is notsubstantially altered. Polypeptide mimetic compositions can contain anycombination of non-natural structural components, which are typicallyfrom three structural groups: a) residue linkage groups other than thenatural amide bond (“peptide bond”) linkages; b) non-natural residues inplace of naturally occurring amino acid residues; or c) residues whichinduce secondary structural mimicry, i.e., to induce or stabilize asecondary structure, e.g., a beta turn, gamma turn, beta sheet, alphahelix conformation, and the like. A polypeptide can be characterized asa mimetic when all or some of its residues are joined by chemical meansother than natural peptide bonds. Individual peptidomimetic residues canbe joined by peptide bonds, other chemical bonds or coupling means, suchas, e.g., glutaraldehyde, N-hydroxysuccinimide esters, bifunctionalmaleimides, N,N′-dicyclohexylcarbodiimide (DCC) orN,N′-diisopropylcarbodiimide (DIC). Linking groups that can be analternative to the traditional amide bond (“peptide bond”) linkagesinclude, e.g., ketomethylene (e.g., —C(═O)—CH₂— for —C(═O)—NH—),aminomethylene (CH₂—NH), ethylene, olefin (CH═CH), ether (CH₂—O),thioether (CH₂—S), tetrazole (CN₄), thiazole, retroamide, thioamide, orester (see, e.g., Spatola, Chemistry and Biochemistry of Amino Acids,Peptides and Proteins, Vol. 7, pp 267-357, “Peptide BackboneModifications,” Marcell Dekker, NY (1983)). A polypeptide can also becharacterized as a mimetic by containing all or some non-naturalresidues in place of naturally occurring amino acid residues;non-natural residues are well described in the scientific and patentliterature.

A “label” or a “detectable moiety” is a composition detectable byspectroscopic, photochemical, biochemical, immunochemical, or chemicalmeans. For example, useful labels include ³²P, fluorescent dyes,electron-dense reagents, enzymes (e.g., as commonly used in an ELISA),biotin, digoxigenin, or haptens and proteins which can be madedetectable, e.g., by incorporating a radiolabel into the peptide or usedto detect antibodies specifically reactive with the peptide.

A “labeled nucleic acid probe or oligonucleotide” is one that is bound,either covalently, through a linker or a chemical bond, ornoncovalently, through ionic, van der Waals, electrostatic, or hydrogenbonds to a label such that the presence of the probe may be detected bydetecting the presence of the label bound to the probe.

As used herein a “nucleic acid probe or oligonucleotide” is defined as anucleic acid capable of binding to a target nucleic acid ofcomplementary sequence through one or more types of chemical bonds,usually through complementary base pairing, usually through hydrogenbond formation. As used herein, a probe may include natural (i.e., A, G,C, or T) or modified bases (7-deazaguanosine, inosine, etc.). Inaddition, the bases in a probe may be joined by a linkage other than aphosphodiester bond, so long as it does not interfere withhybridization. Thus, for example, probes may be peptide nucleic acids inwhich the constituent bases are joined by peptide bonds rather thanphosphodiester linkages. It will be understood by one of skill in theart that probes may bind target sequences lacking completecomplementarity with the probe sequence depending upon the stringency ofthe hybridization conditions. The probes are optionally directly labeledas with isotopes, chromophores, lumiphores, chromogens, or indirectlylabeled such as with biotin to which a streptavidin complex may laterbind. By assaying for the presence or absence of the probe, one candetect the presence or absence of the select sequence or subsequence.

The term “heterologous” when used with reference to portions of anucleic acid indicates that the nucleic acid comprises two or moresubsequences that are not found in the same relationship to each otherin nature. For instance, the nucleic acid is typically recombinantlyproduced, having two or more sequences from unrelated genes arranged tomake a new functional nucleic acid, e.g., a promoter from one source anda coding region from another source. Similarly, a heterologous proteinindicates that the protein comprises two or more subsequences that arenot found in the same relationship to each other in nature (e.g., afusion protein).

A “promoter” is defined as an array of nucleic acid sequences thatdirect transcription of a nucleic acid. As used herein, a promoterincludes necessary nucleic acid sequences near the start site oftranscription, such as, in the case of a polymerase II type promoter, aTATA element. A promoter also optionally includes distal enhancer orrepressor elements, which can be located as much as several thousandbase pairs from the start site of transcription. A “constitutive”promoter is a promoter that is active under most environmental anddevelopmental conditions.

An “inducible” promoter is a promoter that is active under environmentalor developmental regulation. The term “operably linked” refers to afunctional linkage between a nucleic acid expression control sequence(such as a promoter, or array of transcription factor binding sites) anda second nucleic acid sequence, wherein the expression control sequencedirects transcription of the nucleic acid corresponding to the secondsequence.

As used herein, “recombinant” refers to a polynucleotide synthesized orotherwise manipulated in vitro (e.g., “recombinant polynucleotide”), tomethods of using recombinant polynucleotides to produce gene products incells or other biological systems, or to a polypeptide (“recombinantprotein”) encoded by a recombinant polynucleotide. “Recombinant means”also encompass the ligation of nucleic acids having various codingregions or domains or promoter sequences from different sources into anexpression cassette or vector for expression of, e.g., inducible orconstitutive expression of a fusion protein comprising a translocationdomain of the invention and a nucleic acid sequence amplified using aprimer of the invention.

As used herein, a “stable cell line” refers to a cell line, whichstably, i.e. over a prolonged period, expresses a heterologous nucleicsequence, i.e. a T1R or G protein. In preferred embodiments, such stablecell lines will be produced by transfecting appropriate cells, typicallymammalian cells, e.g. HEK-293 cells, with a linearized vector thatcontains a T1R expression construct, i.e. T1R1, T1R2 and/or T1R3. Mostpreferably, such stable cell lines will be produced by co-transfectingtwo linearized plasmids that express hT1R1 and hT1R3 or hT1R2 and hT1R3and an appropriate selection procedure to generate cell lines havingthese genes stably integrated therein. Most preferably, the cell linewill also stably express a G protein such as G_(α15).

The phrase “selectively (or specifically) hybridizes to” refers to thebinding, duplexing, or hybridizing of a molecule only to a particularnucleotide sequence under stringent hybridization conditions when thatsequence is present in a complex mixture (e.g., total cellular orlibrary DNA or RNA).

The phrase “stringent hybridization conditions” refers to conditionsunder which a probe will hybridize to its target subsequence, typicallyin a complex mixture of nucleic acid, but to no other sequences.Stringent conditions are sequence dependent and will be different indifferent circumstances. Longer sequences hybridize specifically athigher temperatures. An extensive guide to the hybridization of nucleicacids is found in Tijssen, Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Probes, “Overview of principles ofhybridization and the strategy of nucleic acid assays” (1993).Generally, stringent conditions are selected to be about 5-10° C. lowerthan the thermal melting point (Tm) for the specific sequence at adefined ionic strength pH. The Tm is the temperature (under definedionic strength, pH, and nucleic concentration) at which 50% of theprobes complementary to the target hybridize to the target sequence atequilibrium (as the target sequences are present in excess, at Tm, 50%of the probes are occupied at equilibrium). Stringent conditions will bethose in which the salt concentration is less than about 1.0 M sodiumion, typically about 0.01 to 1.0 M sodium ion concentration (or othersalts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. forshort probes (e.g., 10 to 50 nucleotides) and at least about 60° C. forlong probes (e.g., greater than 50 nucleotides). Stringent conditionsmay also be achieved with the addition of destabilizing agents such asformamide. For selective or specific hybridization, a positive signal isat least two times background, optionally 10 times backgroundhybridization. Exemplary stringent hybridization conditions can be asfollowing: 50% formamide, 5×SSC, and 1% SDS, incubating at 42° C., or,5×SSC, 1% SDS, incubating at 65° C., with wash in 0.2×SSC, and 0.1% SDSat 65° C. Such hybridizations and wash steps can be carried out for,e.g., 1, 2, 5, 10, 15, 30, 60; or more minutes.

Nucleic acids that do not hybridize to each other under stringentconditions are still substantially related if the polypeptides that theyencode are substantially related. This occurs, for example, when a copyof a nucleic acid is created using the maximum codon degeneracypermitted by the genetic code. In such cases, the nucleic acidstypically hybridize under moderately stringent hybridization conditions.Exemplary “moderately stringent hybridization conditions” include ahybridization in a buffer of 40% formamide, 1 M NaCl, 1% SDS at 37° C.,and a wash in 1×SSC at 45° C. Such hybridizations and wash steps can becarried out for, e.g., 1, 2, 5, 10, 15, 30, 60, or more minutes. Apositive hybridization is at least twice background. Those of ordinaryskill will readily recognize that alternative hybridization and washconditions can be utilized to provide conditions of similar stringency.

“Antibody” refers to a polypeptide comprising a framework region from animmunoglobulin gene or fragments thereof that specifically binds andrecognizes an antigen. The recognized immunoglobulin genes include thekappa, lambda, alpha, gamma, delta, epsilon, and mu constant regiongenes, as well as the myriad immunoglobulin variable region genes. Lightchains are classified as either kappa or lambda. Heavy chains areclassified as gamma, mu, alpha, delta, or epsilon, which in turn definethe immunoglobulin classes, IgG, IgM, IgA, IgD and IgE, respectively.

An exemplary immunoglobulin (antibody) structural unit comprises atetramer. Each tetramer is composed of two identical pairs ofpolypeptide chains, each pair having one “light” (about 25 kDa) and one“heavy” chain (about 50-70 kDa). The N-terminus of each chain defines avariable region of about 100 to 110 or more amino acids primarilyresponsible for antigen recognition. The terms “variable light chain”(VL) and “variable heavy chain” (VH) refer to these light and heavychains respectively.

A “chimeric antibody” is an antibody molecule in which (a) the constantregion, or a portion thereof, is altered, replaced or exchanged so thatthe antigen binding site (variable region) is linked to a constantregion of a different or altered class, effector function and/orspecies, or an entirely different molecule which confers new propertiesto the chimeric antibody, e.g., an enzyme, toxin, hormone, growthfactor, drug, etc.; or (b) the variable region, or a portion thereof, isaltered, replaced or exchanged with a variable region having a differentor altered antigen specificity.

An “anti-T1R” antibody is an antibody or antibody fragment thatspecifically binds a polypeptide encoded by a T1R gene, cDNA, or asubsequence thereof.

The term “immunoassay” is an assay that uses an antibody to specificallybind an antigen. The immunoassay is characterized by the use of specificbinding properties of a particular antibody to isolate, target, and/orquantify the antigen.

The phrase “specifically (or selectively) binds” to an antibody or,“specifically (or selectively) immunoreactive with,” when referring to aprotein or peptide, refers to a binding reaction that is determinativeof the presence of the protein in a heterogeneous population of proteinsand other biologics. Thus, under designated immunoassay conditions, thespecified antibodies bind to a particular protein at least two times thebackground and do not substantially bind in a significant amount toother proteins present in the sample. Specific binding to an antibodyunder such conditions may require an antibody that is selected for itsspecificity for a particular protein. For example, polyclonal antibodiesraised to a T1R family member from specific species such as rat, mouse,or human can be selected to obtain only those polyclonal antibodies thatare specifically immunoreactive with the T1R polypeptide or animmunogenic portion thereof and not with other proteins, except fororthologs or polymorphic variants and alleles of the T1R polypeptide.This selection may be achieved by subtracting out antibodies thatcross-react with T1R molecules from other species or other T1Rmolecules. Antibodies can also be selected that recognize only T1R GPCRfamily members but not GPCRs from other families.

A variety of immunoassay formats may be used to select antibodiesspecifically immunoreactive with a particular protein. For example,solid-phase ELISA immunoassays are routinely used to select antibodiesspecifically immunoreactive with a protein (see, e.g., Harlow & Lane,Antibodies, A Laboratory Manual, (1988), for a description ofimmunoassay formats and conditions that can be used to determinespecific immunoreactivity). Typically a specific or selective reactionwill be at least twice background signal or noise and more typicallymore than 10 to 100 times background.

The phrase “selectively associates with” refers to the ability of anucleic acid to “selectively hybridize” with another as defined above,or the ability of an antibody to “selectively (or specifically) bind toa protein, as defined above.

The term “expression vector” refers to any recombinant expression systemfor the purpose of expressing a nucleic acid sequence of the inventionin vitro or in vivo, constitutively or inducibly, in any cell, includingprokaryotic, yeast, fungal, plant, insect or mammalian cell. The termincludes linear or circular expression systems. The term includesexpression systems that remain episomal or integrate into the host cellgenome. The expression systems can have the ability to self-replicate ornot, i.e., drive only transient expression in a cell. The term includesrecombinant expression “cassettes which contain only the minimumelements needed for transcription of the recombinant nucleic acid.

By “host cell” is meant a cell that contains an expression vector andsupports the replication or expression of the expression vector. Hostcells may be prokaryotic cells such as E. coli, or eukaryotic cells suchas yeast, insect, amphibian, worm or mammalian cells such as CHO, Hela,HEK-293, and the like, e.g., cultured cells, explants, and cells invivo.

Isolation and Expression of T1R Polypeptides

Isolation and expression of the T1Rs, or fragments or variants thereof,of the invention can be performed as described below. PCR primers can beused for the amplification of nucleic acids encoding taste receptorligand-binding regions, and libraries of these nucleic acids canoptionally be generated. Individual expression vectors or libraries ofexpression vectors can then be used to infect or transfect host cellsfor the functional expression of these nucleic acids or libraries. Thesegenes and vectors can be made and expressed in vitro or in vivo. One ofskill will recognize that desired phenotypes for altering andcontrolling nucleic acid expression can be obtained by modulating theexpression or activity of the genes and nucleic acids (e.g., promoters,enhancers and the like) within the vectors of the invention. Any of theknown methods described for increasing or decreasing expression oractivity can be used. The invention can be practiced in conjunction withany method or protocol known in the art, which are well described in thescientific and patent literature.

The nucleic acid sequences of the invention and other nucleic acids usedto practice this invention, whether RNA, cDNA, genomic DNA, vectors,viruses or hybrids thereof, may be isolated from a variety of sources,genetically engineered, amplified, and/or expressed recombinantly. Anyrecombinant expression system can be used, including, in addition tomammalian cells, e.g., bacterial, yeast, insect, or plant systems.

Alternatively, these nucleic acids can be synthesized in vitro bywell-known chemical synthesis techniques, as described in, e.g.,Carruthers, Cold Spring Harbor Symp. Quant. Biol. 47:411-418 (1982);Adams, Am. Chem. Soc. 105:661 (1983); Belousov, Nucleic Acids Res.25:3440-3444 (1997); Frenkel, Free Radic. Biol. Med. 19:373-380 (1995);Blommers, Biochemistry 33:7886-7896 (1994); Narang, Meth. Enzymol. 68:90(1979); Brown, Meth. Enzymol. 68:109 (1979); Beaucage, Tetra. Lett.22:1859 (1981); U.S. Pat. No. 4,458,066. Double-stranded DNA fragmentsmay then be obtained either by synthesizing the complementary strand andannealing the strands together under appropriate conditions, or byadding the complementary strand using DNA polymerase with an appropriateprimer sequence.

Techniques for the manipulation of nucleic acids, such as, for example,for generating mutations in sequences, subcloning, labeling probes,sequencing, hybridization and the like are well described in thescientific and patent literature. See, e.g., Sambrook, ed., MolecularCloning: a Laboratory manual (2nd ed.), Vols. 1-3, Cold Spring HarborLaboratory (1989); Current Protocols in Molecular Biology, Ausubel, ed.John Wiley & Sons, Inc., New York (1997); Laboratory Techniques inBiochemistry and Molecular Biology: Hybridization With Nucleic AcidProbes, Part I, Theory and Nucleic Acid Preparation, Tijssen, ed.Elsevier, N.Y. (1993).

Nucleic acids, vectors, capsids, polypeptides, and the like can beanalyzed and quantified by any of a number of general means well knownto those of skill in the art. These include, e.g., analyticalbiochemical methods such as NMR, spectrophotometry, radiography,electrophoresis, capillary electrophoresis, high performance liquidchromatography (HPLC), thin layer chromatography (TLC), andhyperdiffusion chromatography, various immunological methods, e.g.,fluid or gel precipitin reactions, immunodiffusion,immunoelectrophoresis, radioimmunoassays (RIAs), enzyme-linkedimmunosorbent assays (ELISAs), immuno-fluorescent assays, Southernanalysis, Northern analysis, dot-blot analysis, gel electrophoresis(e.g., SDS-PAGE), RT-PCR, quantitative PCR, other nucleic acid or targetor signal amplification methods, radiolabeling, scintillation counting,and affinity chromatography.

Oligonucleotide primers may be used to amplify nucleic acid fragmentsencoding taste receptor ligand-binding regions. The nucleic acidsdescribed herein can also be cloned or measured quantitatively usingamplification techniques. Amplification methods are also well known inthe art, and include, e.g., polymerase chain reaction, PCR (PCRProtocols, a Guide to Methods and Applications, ed. Innis. AcademicPress, N.Y. (1990) and PCR Strategies, ed. Innis, Academic Press, Inc.,N.Y. (1995), ligase chain reaction (LCR) (see, e.g., Wu, Genomics 4:560(1989); Landegren, Science 241:1077, (1988); Barringer, Gene 89:117(1990)); transcription amplification (see, e.g., Kwoh, Proc. Natl. Acad.Sci. USA 86:1173 (1989)); and, self-sustained sequence replication (see,e.g., Guatelli, Proc. Natl. Acad. Sci. USA 87:1874 (1990)); Q Betareplicase amplification (see, e.g., Smith, J. Clin. Microbiol.35:1477-1491 (1997)); automated Q-beta replicase amplification assay(see, e.g., Burg, Mol. Cell. Probes 10:257-271 (1996)) and other RNApolymerase mediated techniques (e.g., NASBA, Cangene, Mississauga,Ontario); see also Berger, Methods Enzymol. 152:307-316 (1987);Sambrook; Ausubel; U.S. Pat. Nos. 4,683,195 and 4,683,202; Sooknanan,Biotechnology 13:563-564 (1995). The primers can be designed to retainthe original sequence of the “donor” 7-membrane receptor. Alternatively,the primers can encode amino acid residues that are conservativesubstitutions (e.g., hydrophobic for hydrophobic residue, see abovediscussion) or functionally benign substitutions (e.g., do not preventplasma membrane insertion, cause cleavage by peptidase, cause abnormalfolding of receptor, and the like). Once amplified, the nucleic acids,either individually or as libraries, may be cloned according to methodsknown in the art, if desired, into any of a variety of vectors usingroutine molecular biological methods; methods for cloning in vitroamplified nucleic acids are described, e.g., U.S. Pat. No. 5,426,039.

The primer pairs may be designed to selectively amplify ligand-bindingregions of the T1R family members. These regions may vary for differentligands or tastants. Thus, what may be a minimal binding region for onetastant, may be too limiting for a second tastant. Accordingly,ligand-binding regions of different sizes comprising differentextracellular domain structures may be amplified.

Paradigms to design degenerate primer pairs are well known in the art.For example, a COnsensus-DEgenerate Hybrid Oligonucleotide Primer(CODEHOP) strategy computer program is accessible ashttp://blocks.fhcrc.org/codehop.html, and is directly linked from theBlockMaker multiple sequence alignment site for hybrid primer predictionbeginning with a set of related protein sequences, as known tastereceptor ligand-binding regions (see, e.g., Rose, Nucleic Acids Res.26:1628-1635 (1998); Singh, Biotechniques 24:318-319 (1998)).

Means to synthesize oligonucleotide primer pairs are well known in theart. “Natural” base pairs or synthetic base pairs can be used. Forexample, use of artificial nucleobases offers a versatile approach tomanipulate primer sequence and generate a more complex mixture ofamplification products. Various families of artificial nucleobases arecapable of assuming multiple hydrogen bonding orientations throughinternal bond rotations to provide a means for degenerate molecularrecognition. Incorporation of these analogs into a single position of aPCR primer allows for generation of a complex library of amplificationproducts. See, e.g., Hoops, Nucleic Acids Res. 25:4866-4871 (1997).Nonpolar molecules can also be used to mimic the shape of natural DNAbases. A non-hydrogen-bonding shape mimic for adenine can replicateefficiently and selectively against a nonpolar shape mimic for thymine(see, e.g., Morales, Nat. Struct. Biol. 5:950-954 (1998)). For example,two degenerate bases can be the pyrimidine base 6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one or the purine baseN6-methoxy-2,6-diaminopurine (see, e.g., Hill, Proc. Natl. Acad. Sci.USA 95:4258-4263 (1998)). Exemplary degenerate primers of the inventionincorporate the nucleobase analog5′-Dimethoxytrityl-N-benzoyl-2′-deoxy-Cytidine,3′-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite(the term “P” in the sequences, see above). This pyrimidine analoghydrogen bonds with purines, including A and G residues.

Polymorphic variants, alleles, and interspecies homologs that aresubstantially identical to a taste receptor disclosed herein can beisolated using the nucleic acid probes described above. Alternatively,expression libraries can be used to clone T1R polypeptides andpolymorphic variants, alleles, and interspecies homologs thereof, bydetecting expressed homologs immunologically with antisera or purifiedantibodies made against a T1R polypeptide, which also recognize andselectively bind to the T1R homolog.

Nucleic acids that encode ligand-binding regions of taste receptors maybe generated by amplification (e.g., PCR) of appropriate nucleic acidsequences using degenerate primer pairs. The amplified nucleic acid canbe genomic DNA from any cell or tissue or mRNA or cDNA derived fromtaste receptor-expressing cells.

In one embodiment, hybrid protein-coding sequences comprising nucleicacids encoding T1Rs fused to translocation sequences may be constructed.Also provided are hybrid T1Rs comprising the translocation motifs andtastant-binding domains of other families of chemosensory receptors,particularly taste receptors. These nucleic acid sequences can beoperably linked to transcriptional or translational control elements,e.g., transcription and translation initiation sequences, promoters andenhancers, transcription and translation terminators, polyadenylationsequences, and other sequences useful for transcribing DNA into RNA. Inconstitutive of recombinant expression cassettes, vectors, andtransgenics, a promoter fragment can be employed to direct expression ofthe desired nucleic acid in all desired cells or tissues.

In another embodiment, fusion proteins may include C-terminal orN-terminal translocation sequences. Further, fusion proteins cancomprise additional elements, e.g., for protein detection, purification,or other applications. Detection and purification facilitating domainsinclude, e.g., metal chelating peptides such as polyhistidine tracts,histidine-tryptophan modules, or other domains that allow purificationon immobilized metals; maltose binding protein; protein A domains thatallow purification on immobilized immunoglobulin; or the domain utilizedin the FLAGS extension/affinity purification system (Immunex Corp.,Seattle, Wash.).

The inclusion of a cleavable linker sequences such as Factor Xa (see,e.g., Ottavi, Biochimie 80:289-293 (1998)), subtilisin proteaserecognition motif (see, e.g., Polyak, Protein Eng. 10:615-619 (1997));enterokinase (Invitrogen, San Diego, Calif.), and the like, between thetranslocation domain (for efficient plasma membrane expression) and therest of the newly translated polypeptide may be useful to facilitatepurification. For example, one construct can include a polypeptideencoding a nucleic acid sequence linked to six histidine residuesfollowed by a thioredoxin, an enterokinase cleavage site (see, e.g.,Williams, Biochemistry 34:1787-1797 (1995)), and an C-terminaltranslocation domain. The histidine residues facilitate detection andpurification while the enterokinase cleavage site provides a means forpurifying the desired protein(s) from the remainder of the fusionprotein. Technology pertaining to vectors encoding fusion proteins andapplication of fusion proteins are well described in the scientific andpatent literature, see, e.g., Kroll, DNA Cell. Biol. 12:441-53 (1993).

Expression vectors, either as individual expression vectors or aslibraries of expression vectors, comprising the ligand-binding domainencoding sequences may be introduced into a genome or into the cytoplasmor a nucleus of a cell and expressed by a variety of conventionaltechniques, well described in the scientific and patent literature. See,e.g., Roberts, Nature 328:731 (1987); Berger supra; Schneider, ProteinExpr. Purif. 6435:10 (1995); Sambrook; Tijssen; Ausubel. Productinformation from manufacturers of biological reagents and experimentalequipment also provide information regarding known biological methods.The vectors can be isolated from natural sources, obtained from suchsources as ATCC or GenBank libraries, or prepared by synthetic orrecombinant methods.

The nucleic acids can be expressed using expression cassettes, vectorsor viruses which are stably or transiently expressed in cells (e.g.,episomal expression systems). Selection markers can be incorporated intoexpression cassettes and vectors to confer a selectable phenotype ontransformed cells and sequences. For example, selection markers can codefor episomal maintenance and replication such that integration into thehost genome is not required. For example, the marker may encodeantibiotic resistance (e.g., chloramphenicol, kanamycin, G418,blasticidin, hygromycin) or herbicide resistance (e.g., chlorosulfuronor Basta) to permit selection of those cells transformed with thedesired DNA sequences (see, e.g., Blondelet-Rouault, Gene 190:315-317(1997); Aubrecht, J. Pharmacol. Exp. Ther. 281:992-997 (1997)). Becauseselectable marker genes conferring resistance to substrates likeneomycin or hygromycin can only be utilized in tissue culture,chemoresistance genes are also used as selectable markers in vitro andin vivo.

A chimeric nucleic acid sequence may encode a T1R ligand-binding domainwithin any 7-transmembrane polypeptide. Because 7-transmembrane receptorpolypeptides have similar primary sequences and secondary and tertiarystructures, structural domains (e.g., extracellular domain, TM domains,cytoplasmic domain, etc.) can be readily identified by sequenceanalysis. For example, homology modeling, Fourier analysis and helicalperiodicity detection can identify and characterize the seven domainswith a 7-transmembrane receptor sequence. Fast Fourier Transform (FFT)algorithms can be used to assess the dominant periods that characterizeprofiles of the hydrophobicity and variability of analyzed sequences.Periodicity detection enhancement and alpha helical periodicity indexcan be done as by, e.g., Donnelly, Protein Sci. 2:55-70 (1993). Otheralignment and modeling algorithms are well known in the art, see, e.g.,Peitsch, Receptors Channels 4:161-164 (1996); Kyte & Doolittle, J. Med.Bio., 157:105-132 (1982); Cronet, Protein Eng. 6:59-64 (1993).

The present invention also includes not only the DNA and proteins havingthe specified nucleic and amino acid sequences, but also DNA fragments,particularly fragments of, e.g., 40, 60, 80, 100, 150, 200, or 250nucleotides, or more, as well as protein fragments of, e.g., 10, 20, 30,50, 70, 100, or 150 amino acids, or more. Optionally, the nucleic acidfragments can encode an antigenic polypeptide, which is capable ofbinding to an antibody raised against a T1R family member. Further, aprotein fragment of the invention can optionally be an antigenicfragment, which is capable of binding to an antibody raised against aT1R family member.

Also contemplated are chimeric proteins, comprising at least 10, 20, 30,50, 70, 100, or 150 amino acids, or more, of one of at least one of theT1R polypeptides described herein, coupled to additional amino acidsrepresenting all or part of another GPCR, preferably a member of the 7transmembrane superfamily. These chimeras can be made from the instantreceptors and another GPCR, or they can be made by combining two or moreof the present T1R receptors. In one embodiment, one portion of thechimera corresponds to or is derived from the extracellular domain of aT1R polypeptide of the invention. In another embodiment, one portion ofthe chimera corresponds to, or is derived from the extracellular domainand one or more of the transmembrane domains of a T1R polypeptidedescribed herein, and the remaining portion or portions can come fromanother GPCR. Chimeric receptors are well known in the art, and thetechniques for creating them and the selection and boundaries of domainsor fragments of G protein-coupled receptors for incorporation thereinare also well known. Thus, this knowledge of those skilled in the artcan readily be used to create such chimeric receptors. The use of suchchimeric receptors can provide, for example, a taste selectivitycharacteristic of one of the receptors specifically disclosed herein,coupled with the signal transduction characteristics of anotherreceptor, such as a well known receptor used in prior art assay systems.

As noted above, such chimeras, analogous to the native T1R receptor, ornative T1R receptor combination or association will bind to and/or beactivated by molecules that normally affect sweet taste or umami taste.Functional chimeric T1R receptors or receptor combinations are moleculeswhich when expressed alone or in combination with other T1Rs or otherGPCRs (which may themselves be chimeric) bind to or which are activatedby taste stimuli, particularly sweet (T1R2/3) or umami taste stimuli(T1R1/3). Molecules that elicit sweet taste include natural andartificial sweeteners such as sucrose, aspartame, xylitol, cyclamate, etal., Molecules that elicit umami taste include glutamate and glutamateanalogs and other compounds that bind to native T1R1 and/or T1R3, suchas 5′-nucleotides.

For example, a domain such as a ligand-binding domain, an extracellulardomain, a transmembrane domain, a transmembrane domain, a cytoplasmicdomain, an N-terminal domain, a C-terminal domain, or any combinationthereof, can be covalently linked to a heterologous protein. Forinstance, an T1R extracellular domain can be linked to a heterologousGPCR transmembrane domain, or a heterologous GPCR extracellular domaincan be linked to a T1R transmembrane domain. Other heterologous proteinsof choice can be used; e.g., green fluorescent protein.

Also within the scope of the invention are host cells for expressing theT1Rs, fragments, chimeras or variants of the invention. To obtain highlevels of expression of a cloned gene or nucleic acid, such as cDNAsencoding the T1Rs, fragments, or variants of the invention, one of skilltypically subclones the nucleic acid sequence of interest into anexpression vector that contains a strong promoter to directtranscription, a transcription/translation terminator, and if for anucleic acid encoding a protein, a ribosome binding site fortranslational initiation. Suitable bacterial promoters are well known inthe art and described, e.g., in Sambrook et al. However, bacterial oreukaryotic expression systems can be used.

Any of the well-known procedures for introducing foreign nucleotidesequences into host cells may be used. These include the use of calciumphosphate transfection, polybrene, protoplast fusion, electroporation,liposomes, microinjection, plasma vectors, viral vectors and any of theother well known methods for introducing cloned genomic DNA, cDNA,synthetic DNA or other foreign genetic material into a host cell (see,e.g., Sambrook et al.) It is only necessary that the particular geneticengineering procedure used be capable of successfully introducing atlest one nucleic acid molecule into the host cell capable of expressingthe T1R, fragment, or variant of interest.

After the expression vector is introduced into the cells, thetransfected cells are cultured under conditions favoring expression ofthe receptor, fragment, or variant of interest, which is then recoveredfrom the culture using standard techniques. Examples of such techniquesare well known in the art. See, e.g., WO 00/06593, which is incorporatedby reference in a manner consistent with this disclosure.

Detection of T1R Polypeptides

In addition to the detection of T1R genes and gene expression usingnucleic acid hybridization technology, one can also use immunoassays todetect T1Rs, e.g., to identify taste receptor cells, and variants of T1Rfamily members. Immunoassays can be used to qualitatively orquantitatively analyze the T1Rs. A general overview of the applicabletechnology can be found in Harlow & Lane, Antibodies: A LaboratoryManual (1988).

1. Antibodies to T1R Family Members

Methods of producing polyclonal and monoclonal antibodies that reactspecifically with a T1R family member are known to those of skill in theart (see, e.g., Coligan, Current Protocols in Immunology (1991); Harlow& Lane, supra; Goding, Monoclonal Antibodies: Principles and Practice(2d ed. 1986); and Kohler & Milstein, Nature, 256:495-497 (1975)). Suchtechniques include antibody preparation by selection of antibodies fromlibraries of recombinant antibodies in phage or similar vectors, as wellas preparation of polyclonal and monoclonal antibodies by immunizingrabbits or mice (see, e.g., Huse et al., Science, 246:1275-1281 (1989);Ward et al., Nature, 341:544-546 (1989)).

A number of T1R-comprising immunogens may be used to produce antibodiesspecifically reactive with a T1R family member. For example, arecombinant T1R polypeptide, or an antigenic fragment thereof, can beisolated as described herein. Suitable antigenic regions include, e.g.,the consensus sequences that are used to identify members of the T1Rfamily. Recombinant proteins can be expressed in eukaryotic orprokaryotic cells as described above, and purified as generallydescribed above. Recombinant protein is the preferred immunogen for theproduction of monoclonal or polyclonal antibodies. Alternatively, asynthetic peptide derived from the sequences disclosed herein andconjugated to a carrier protein can be used an immunogen. Naturallyoccurring protein may also be used either in pure or impure form. Theproduct is then injected into an animal capable of producing antibodies.Either monoclonal or polyclonal antibodies may be generated, forsubsequent use in immunoassays to measure the protein.

Methods of production of polyclonal antibodies are known to those ofskill in the art. For example, an inbred strain of mice (e.g., BALB/Cmice) or rabbits is immunized with the protein using a standardadjuvant, such as Freund's adjuvant, and a standard immunizationprotocol. The animal's immune response to the immunogen preparation ismonitored by taking test bleeds and determining the titer of reactivityto the T1R. When appropriately high titers of antibody to the immunogenare obtained, blood is collected from the animal and antisera areprepared. Further fractionation of the antisera to enrich for antibodiesreactive to the protein can be done if desired (see Harlow & Lane,supra).

Monoclonal antibodies may be obtained by various techniques familiar tothose skilled in the art. Briefly, spleen cells from an animal immunizedwith a desired antigen may be immortalized, commonly by fusion with amyeloma cell (see Kohler & Milstein, Eur. J. Immunol., 6:511-519(1976)). Alternative methods of immortalization include transformationwith Epstein Barr Virus, oncogenes, or retroviruses, or other methodswell known in the art. Colonies arising from single immortalized cellsare screened for production of antibodies of the desired specificity andaffinity for the antigen, and yield of the monoclonal antibodiesproduced by such cells may be enhanced by various techniques, includinginjection into the peritoneal cavity of a vertebrate host.Alternatively, one may isolate DNA sequences which encode a monoclonalantibody or a binding fragment thereof by screening a DNA library fromhuman B cells according to the general protocol outlined by Huse et al.,Science, 246:1275-1281 (1989).

Monoclonal antibodies and polyclonal sera are collected and titeredagainst the immunogen protein in an immunoassay, for example, a solidphase immunoassay with the immunogen immobilized on a solid support.Typically, polyclonal antisera with a titer of 104 or greater areselected and tested for their cross reactivity against non-T1Rpolypeptides, or even other T1R family members or other related proteinsfrom other organisms, using a competitive binding immunoassay. Specificpolyclonal antisera and monoclonal antibodies will usually bind with aKd of at least about 0.1 mM, more usually at least about 1 pM,optionally at least about 0.1 pM or better, and optionally 0.01 pM orbetter.

Once T1R family member specific antibodies are available, individual T1Rproteins and protein fragments can be detected by a variety ofimmunoassay methods. For a review of immunological and immunoassayprocedures, see Basic and Clinical Immunology (Stites & Terr eds., 7thed. 1991). Moreover, the immunoassays of the present invention can beperformed in any of several configurations, which are reviewedextensively in Enzyme Immunoassay (Maggio, ed., 1980); and Harlow &Lane, supra.

2. Immunological Binding Assays

T1R proteins, fragments, and variants can be detected and/or quantifiedusing any of a number of well-recognized immunological binding assays(see, e.g., U.S. Pat. Nos. 4,366,241; 4,376,110; 4,517,288; and4,837,168). For a review of the general immunoassays, see also Methodsin Cell Biology: Antibodies in Cell Biology, volume 37 (Asai, ed. 1993);Basic and Clinical Immunology (Stites & Terr, eds., 7th ed. 1991).Immunological binding assays (or immunoassays) typically use an antibodythat specifically binds to a protein or antigen of choice (in this casea T1R family member or an antigenic subsequence thereof). The antibody(e.g., anti-T1R) may be produced by any of a number of means well knownto those of skill in the art and as described above.

Immunoassays also often use a labeling agent to specifically bind to andlabel the complex formed by the antibody and antigen. The labeling agentmay itself be one of the moieties comprising the antibody/antigencomplex. Thus, the labeling agent may be a labeled T1R polypeptide or alabeled anti-T1R antibody. Alternatively, the labeling agent may be athird moiety, such a secondary antibody, that specifically binds to theantibody/T1R complex (a secondary antibody is typically specific toantibodies of the species from which the first antibody is derived).Other proteins capable of specifically binding immunoglobulin constantregions, such as protein A or protein G may also be used as the labelagent. These proteins exhibit a strong non-immunogenic reactivity withimmunoglobulin constant regions from a variety of species (see, e.g.,Kronval et al., J. Immunol., 111:1401-1406 (1973); Akerstrom et al., J.Immunol., 135:2589-2542 (1985)). The labeling agent can be modified witha detectable moiety, such as biotin, to which another molecule canspecifically bind, such as streptavidin. A variety of detectablemoieties are well known to those skilled in the art.

Throughout the assays, incubation and/or washing steps may be requiredafter each combination of reagents. Incubation steps can vary from about5 seconds to several hours, optionally from about 5 minutes to about 24hours. However, the incubation time will depend upon the assay format,antigen, volume of solution, concentrations, and the like. Usually, theassays will be carried out at ambient temperature, although they can beconducted over a range of temperatures, such as 10° C. to 40° C.

A. Non-competitive Assay Formats

Immunoassays for detecting a T1R polypeptide in a sample may be eithercompetitive or noncompetitive. Noncompetitive immunoassays are assays inwhich the amount of antigen is directly measured. In one preferred“sandwich” assay, for example, the anti-T1R antibodies can be bounddirectly to a solid substrate on which they are immobilized. Theseimmobilized antibodies then capture the T1R polypeptide present in thetest sample. The T1R polypeptide is thus immobilized is then bound by alabeling agent, such as a second T1R antibody bearing a label.Alternatively, the second antibody may lack a label, but it may, inturn, be bound by a labeled third antibody specific to antibodies of thespecies from which the second antibody is derived. The second or thirdantibody is typically modified with a detectable moiety, such as biotin,to which another molecule specifically binds, e.g., streptavidin, toprovide a detectable moiety.

B. Competitive Assay Formats

In competitive assays, the amount of T1R polypeptide present in thesample is measured indirectly by measuring the amount of a known, added(exogenous) T1R polypeptide displaced (competed away) from an anti-T1Rantibody by the unknown T1R polypeptide present in a sample. In onecompetitive assay, a known amount of T1R polypeptide is added to asample and the sample is then contacted with an antibody thatspecifically binds to the T1R. The amount of exogenous T1R polypeptidebound to the antibody is inversely proportional to the concentration ofT1R polypeptide present in the sample. In a particularly preferredembodiment, the antibody is immobilized on a solid substrate. The amountof T1R polypeptide bound to the antibody may be determined either bymeasuring the amount of T1R polypeptide present in a T1R/antibodycomplex, or alternatively by measuring the amount of remaininguncomplexed protein. The amount of T1R polypeptide may be detected byproviding a labeled T1R molecule.

A hapten inhibition assay is another preferred competitive assay. Inthis assay the known T1R polypeptide is immobilized on a solidsubstrate. A known amount of anti-T1R antibody is added to the sample,and the sample is then contacted with the immobilized T1R. The amount ofanti-T1R antibody bound to the known immobilized T1R polypeptide isinversely proportional to the amount of T1R polypeptide present in thesample. Again, the amount of immobilized antibody may be detected bydetecting either the immobilized fraction of antibody or the fraction ofthe antibody that remains in solution. Detection may be direct where theantibody is labeled or indirect by the subsequent addition of a labeledmoiety that specifically binds to the antibody as described above.

C. Cross-reactivity Determinations

Immunoassays in the competitive binding format can also be used forcross-reactivity determinations. For example, a protein at leastpartially encoded by the nucleic acid sequences disclosed herein can beimmobilized to a solid support. Proteins (e.g., T1R polypeptides andhomologs) are added to the assay that compete for binding of theantisera to the immobilized antigen. The ability of the added proteinsto compete for binding of the antisera to the immobilized protein iscompared to the ability of the T1R polypeptide encoded by the nucleicacid sequences disclosed herein to compete with itself. The percentcross-reactivity for the above proteins is calculated, using standardcalculations. Those antisera with less than 10% cross-reactivity witheach of the added proteins listed above are selected and pooled. Thecross-reacting antibodies are optionally removed from the pooledantisera by immunoabsorption with the added considered proteins, e.g.,distantly related homologs. In addition, peptides comprising amino acidsequences representing conserved motifs that are used to identifymembers of the T1R family can be used in cross-reactivitydeterminations.

The immunoabsorbed and pooled antisera are then used in a competitivebinding immunoassay as described above to compare a second protein,thought to be perhaps an allele or polymorphic variant of a T1R familymember, to the immunogen protein (i.e., T1R polypeptide encoded by thenucleic acid sequences disclosed herein). In order to make thiscomparison, the two proteins are each assayed at a wide range ofconcentrations and the amount of each protein required to inhibit 50% ofthe binding of the antisera to the immobilized protein is determined. Ifthe amount of the second protein required to inhibit 50% of binding isless than 10 times the amount of the protein encoded by nucleic acidsequences disclosed herein required to inhibit 50% of binding, then thesecond protein is said to specifically bind to the polyclonal antibodiesgenerated to a T1R immunogen.

Antibodies raised against T1R conserved motifs can also be used toprepare antibodies that specifically bind only to GPCRs of the T1Rfamily, but not to GPCRs from other families.

Polyclonal antibodies that specifically bind to a particular member ofthe T1R family can be made by subtracting out cross-reactive antibodiesusing other T1R family members. Species-specific polyclonal antibodiescan be made in a similar way. For example, antibodies specific to humanT1R1 can be made by, subtracting out antibodies that are cross-reactivewith orthologous sequences, e.g., rat T1R1 or mouse T1R1.

D. Other Assay Formats

Western blot (immunoblot) analysis is used to detect and quantify thepresence of T1R polypeptide in the sample. The technique generallycomprises separating sample proteins by gel electrophoresis on the basisof molecular weight, transferring the separated proteins to a suitablesolid support, (such as a nitrocellulose filter, a nylon filter, orderivatized nylon filter), and incubating the sample with the antibodiesthat specifically bind the T1R polypeptide. The anti-T1R polypeptideantibodies specifically bind to the T1R polypeptide on the solidsupport. These antibodies may be directly labeled or alternatively maybe subsequently detected using labeled antibodies (e.g., labeled sheepanti-mouse antibodies) that specifically bind to the anti-T1Rantibodies.

Other, assay formats include liposome immunoassays (LIA), which useliposomes designed to bind specific molecules (e.g., antibodies) andrelease encapsulated reagents or markers. The released chemicals arethen detected according to standard techniques (see Monroe et al., Amer.Clin. Prod. Rev., 5:34-41 (1986)).

E. Reduction of Non-specific Binding

One of skill in the art will appreciate that it is often desirable tominimize non-specific binding in immunoassays. Particularly, where theassay involves an antigen or antibody immobilized on a solid substrateit is desirable to minimize the amount of non-specific binding to thesubstrate. Means of reducing such non-specific binding are well known tothose of skill in the art. Typically, this technique involves coatingthe substrate with a proteinaceous composition. In particular, proteincompositions such as bovine serum albumin (BSA), nonfat powdered milk,and gelatin are widely used with powdered milk being most preferred.

F. Labels

The particular label or detectable group used in the assay is not acritical aspect of the invention, as long as it does not significantlyinterfere with the specific binding of the antibody used in the assay.The detectable group can be any material having a detectable physical orchemical property. Such detectable labels have been well developed inthe field of immunoassays and, in general, most any label useful in suchmethods can be applied to the present invention. Thus, a label is anycomposition detectable by spectroscopic, photochemical, biochemical,immunochemical, electrical, optical, or chemical means. Useful labels inthe present invention include magnetic beads (e.g., DYNABEADS™),fluorescent dyes (e.g., fluorescein isothiocyanate, Texas red,rhodamine, and the like), radiolabels (e.g., ³H, ¹²⁵I, ¹⁴C, ³⁵S),enzymes (e.g., horseradish peroxidase, alkaline phosphates and otherscommonly used in an ELISA), and colorimetric labels such as colloidalgold or colored glass or plastic beads (e.g., polystyrene,polypropylene, latex, etc.).

The label may be coupled directly or indirectly to the desired componentof the assay according to methods well known in the art. As indicatedabove, a wide variety of labels may be used, with the choice of labeldepending on sensitivity required, ease of conjugation with thecompound, stability requirements, available instrumentation, anddisposal provisions.

Non-radioactive labels are, often attached by indirect means. Generally,a ligand molecule (e.g., biotin) is covalently bound to the molecule.The ligand then binds to another molecules (e.g., streptavidin)molecule, which is either inherently detectable or covalently bound to asignal system, such as a detectable enzyme, a fluorescent compound, or achemiluminescent compound. The ligands and their targets can be used inany suitable combination with antibodies that recognize a T1Rpolypeptide, or secondary antibodies that recognize anti-T1R.

The molecules can also be conjugated directly to signal generatingcompounds, e.g., by conjugation with an enzyme or fluorophore. Enzymesof interest as labels will primarily be hydrolases, particularlyphosphatases, esterases and glycosidases, or oxidotases, particularlyperoxidases. Fluorescent compounds include fluorescein and itsderivatives, rhodamine and its derivatives, dansyl, umbelliferone, etc.Chemiluminescent compounds include luciferin, and2,3-dihydrophthalazinediones, e.g., luminol. For a review of variouslabeling or signal producing systems that may be used, see U.S. Pat. No.4,391,904.

Means of detecting labels are well known to those of skill in the art.Thus, for example, where the label is a radioactive label, means fordetection include a scintillation counter or photographic film as inautoradiography. Where the label is a fluorescent label, it may bedetected by exciting the fluorochrome with the appropriate wavelength oflight and detecting the resulting fluorescence. The fluorescence may bedetected visually, by means of photographic film, by the use ofelectronic detectors such as charge-coupled devices (CCDs) orphotomultipliers and the like. Similarly, enzymatic labels may bedetected by providing the appropriate substrates for the enzyme anddetecting the resulting reaction product. Finally simple colorimetriclabels may be detected simply by observing the color associated with thelabel. Thus, in various dipstick assays, conjugated gold often appearspink, while various conjugated beads appear the color of the bead.

Some assay formats do not require the use of labeled components. Forinstance, agglutination assays can be used to detect the presence of thetarget antibodies. In this case, antigen-coated particles areagglutinated by samples comprising the target antibodies. In thisformat, none of the components need be labeled and the presence of thetarget antibody is detected by simple visual inspection.

Detection of Modulators

Compositions and methods for determining whether a test compoundspecifically binds to a T1R receptor of the invention, both in vitro andin vivo, are described below. Many aspects of cell physiology can bemonitored to assess the effect of ligand binding to a T1R polypeptide ofthe invention. These assays may be performed on intact cells expressinga chemosensory receptor, on permeabilized cells, or on membranefractions produced by standard methods or in vitro de novo synthesizedproteins.

In vivo, taste receptors bind tastants and initiate the transduction ofchemical stimuli into electrical signals. An activated or inhibited Gprotein will in turn alter the properties of target enzymes, channels,and other effector proteins. Some examples are the activation of cGMPphosphodiesterase by transducin in the visual system, adenylate cyclaseby the stimulatory G protein, phospholipase C by Gq and other cognate Gproteins, and modulation of diverse channels by Gi and other G proteins.Downstream consequences can also be examined such as generation ofdiacyl glycerol and IP3 by phospholipase C, and in turn, for calciummobilization by IP3.

The T1R proteins or polypeptides of the assay will preferably beselected from a polypeptide having the T1R polypeptide sequence selectedfrom those disclosed in Example 1, or fragments or conservativelymodified variants thereof. Optionally, the fragments and variants can beantigenic fragments and variants which bind to an anti-T1R antibody.Optionally, the fragments and variants can bind to or are activated bysweeteners or umami tastants.

Alternatively, the T1R proteins or polypeptides of the assay can bederived from a eukaryotic host cell and can include an amino acidsubsequence having amino acid sequence identity to the T1R polypeptidesdisclosed in Example 1, or fragments or conservatively modified variantsthereof. Generally, the amino acid sequence identity will be at least 35to 50%, or optionally 75%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%.Optionally, the T1R proteins or polypeptides of the assays can comprisea domain of a T1R protein, such as an extracellular domain,transmembrane region, transmembrane domain, cytoplasmic domain,ligand-binding domain, and the like. Further, as described above, theT1R protein or a domain thereof can be covalently linked to aheterologous protein to create a chimeric protein used in the assaysdescribed herein.

Modulators of T1R receptor activity are tested using T1R proteins orpolypeptides as described above, either recombinant or naturallyoccurring. The T1R proteins or polypeptides can be isolated,co-expressed in a cell, co-expressed in a membrane derived from a cell,co-expressed in tissue or in an animal, either recombinant or naturallyoccurring. For example, tongue slices, dissociated cells from a tongue,transformed cells, or membranes can be used. Modulation can be testedusing one of the in vitro or in vivo assays described herein.

For example, as disclosed in the experiment examples infra, it has beendiscovered that certain 5¹ nucleotides, e.g., 5¹ IMP or 5¹ GMP, enhancethe activity of L-glutamate to activate the umami taste receptor, orblock the activation of the umami taste receptor by umami taste stimulisuch as L-glutamate and L-aspartate.

1. In vitro Binding Assays

Taste transduction can also be examined in vitro with soluble or solidstate reactions, using the T1R polypeptides of the invention. In aparticular embodiment, T1R ligand-binding domains can be used in vitroin soluble or solid state reactions to assay for ligand binding.

For instance, the T1R N-terminal domain is predicted to be involved inligand binding. More particularly, the T1Rs belong to a GPCR sub-familythat is characterized by large, approximately 600 amino acid,extracellular N-terminal segments. These N-terminal segments are thoughtto form the ligand-binding domains, and are therefore useful inbiochemical assays to identify T1R agonists and antagonists. It ispossible that the ligand-binding domain may be formed by additionalportions of the extracellular domain, such as the extracellular loops ofthe transmembrane domain.

In vitro binding assays have been used with other GPCRs that are relatedto the T1Rs, such as the metabotropic glutamate receptors (see, e.g.,Han and Hampson, J. Biol. Chem. 274:10008-10013 (1999)). These assaysmight involve displacing a radioactively or fiuorescently labeledligand, measuring changes in intrinsic fluorescence or changes inproteolytic susceptibility, etc.

Ligand binding to a hetero-multimeric complex of T1R polypeptides of theinvention can be tested in solution, in a bilayer membrane, optionallyattached to a solid phase, in a lipid monolayer, or in vesicles. Bindingof a modulator can be tested using, e.g., changes in spectroscopiccharacteristics (e.g., fluorescence, absorbence, refractive index)hydrodynamic (e.g., shape), chromatographic, or solubility properties.

In another embodiment of the invention, a GTPγ³⁵S assay may be used. Asdescribed above, upon activation of a GPCR, the Gα subunit of the Gprotein complex is stimulated to exchange bound GDP for GTP.Ligand-mediated stimulation of G protein exchange activity can bemeasured in a biochemical assay measuring the binding of addedradioactively labeled GTPγ³⁵S to the G protein in the presence of aputative ligand. Typically, membranes containing the chemosensoryreceptor of interest are mixed with a complex of G proteins. Potentialinhibitors and/or activators and GTPγ³⁵S are added to the assay, andbinding of GTPγ³⁵S to the G protein is measured. Binding can be measuredby liquid scintillation counting or by any other means known in the art,including scintillation proximity assays (SPA). In other assays formats,fluorescently labeled GTPγS can be utilized.

2. Fluorescence Polarization Assays

In another embodiment, Fluorescence Polarization (“FP”) based assays maybe used to detect and monitor ligand binding. Fluorescence polarizationis a versatile laboratory technique for measuring equilibrium binding,nucleic acid hybridization, and enzymatic activity. Fluorescencepolarization assays are homogeneous in that they do not require aseparation step such as centrifugation, filtration, chromatography,precipitation, or electrophoresis. These assays are done in real time,directly in solution and do not require an immobilized phase.Polarization values can be measured repeatedly and after the addition ofreagents since measuring the polarization is rapid and does not destroythe sample. Generally, this technique can be used to measurepolarization values of fluorophores from low picomolar to micromolarlevels. This section describes how fluorescence polarization can be usedin a simple and quantitative way to measure the binding of ligands tothe T1R polypeptides of the invention.

When a fluorescently labeled molecule is excited with plane-polarizedlight, it emits light that has a degree of polarization that isinversely proportional to its molecular rotation. Large fluorescentlylabeled molecules remain relatively stationary during the excited state(4 nanoseconds in the case of fluorescein) and the polarization of thelight remains relatively constant between excitation and emission. Smallfluorescently labeled molecules rotate rapidly during the excited stateand the polarization changes significantly between excitation andemission. Therefore, small molecules have low polarization values andlarge molecules have high polarization values. For example, asingle-stranded fluorescein-labeled oligonucleotide has a relatively lowpolarization value but when it is hybridized to a complementary strand,it has a higher polarization value. When using FP to detect and monitortastant-binding which may activate or inhibit the chemosensory receptorsof the invention, fluorescence-labeled tastants or auto-fluorescenttastants may be used.

Fluorescence polarization (P) is defined as:

$P = \frac{{Int}_{\coprod} - {Int}_{\bot}}{{Int}_{\coprod} + {Int}_{\bot}}$Where Π is the intensity of the emission light parallel to theexcitation light plane and Int ⊥ is the intensity of the emission lightperpendicular to the excitation light plane. P, being a ratio of lightintensities, is a dimensionless number. For example, the Beacon® andBeacon 2000™ System may be used in connection with these assays. Suchsystems typically express polarization in millipolarization units (1Polarization Unit=1000 mP Units).

The relationship between molecular rotation and size is described by thePerrin equation and the reader is referred to Jolley, M. E. (1991) inJournal of Analytical Toxicology, pp. 236-240, which gives a thoroughexplanation of this equation. Summarily, the Perrin equation states thatpolarization is directly proportional to the rotational relaxation time,the time that it takes a molecule to rotate through an angle ofapproximately 68.5° Rotational relaxation time is related to viscosity(η), absolute temperature (T), molecular volume (V), and the gasconstant (R) by the following equation:

${{Rotational}{\mspace{11mu}\;}{Relaxation}\mspace{14mu}{Time}} = \frac{3\eta\; V}{RT}$

The rotational relaxation time is small (≈1 nanosecond) for smallmolecules (e.g. fluorescein) and large (≈100 nanoseconds) for largemolecules (e.g. immunoglobulins). If viscosity and temperature are heldconstant, rotational relaxation time, and therefore polarization, isdirectly related to the molecular volume. Changes in molecular volumemay be due to interactions with other molecules, dissociation,polymerization, degradation, hybridization, or conformational changes ofthe fluorescently labeled molecule. For example, fluorescencepolarization has been used to measure enzymatic cleavage of largefluorescein labeled polymers by proteases, DNases, and RNases. It alsohas been used to measure equilibrium binding for protein/proteininteractions, antibody/antigen binding, and protein/DNA binding.

A. Solid State and Soluble High Throughput Assays

In yet another embodiment, the invention provides soluble assays using ahetero-oligomeric T1R polypeptide complex; or a cell or tissueco-expressing T1R polypeptides. Preferably, the cell will comprise acell line that stably co-expresses a functional T1R1/T1R3 (umami) tastereceptor or T1R2/T1R3 (sweet) taste receptor. In another embodiment, theinvention provides solid phase based in vitro assays in a highthroughput format, where the T1R polypeptides, or cell or tissueexpressing the T1R polypeptides is attached to a solid phase substrateor a taste stimulating compound and contacted with a T1R receptor, andbinding detected using an appropriate tag or antibody raised against theT1R receptor.

In the high throughput assays of the invention, it is possible to screenup to several thousand different modulators or ligands in a single day.In particular, each well of a microtiter plate can be used to run aseparate assay against a selected potential modulator, or, ifconcentration or incubation time effects are to be observed, every 5-10wells can test a single modulator. Thus, a single standard microtiterplate can assay about 100 (e.g., 96) modulators. If 1536 well plates areused, then a single plate can easily assay from about 1000 to about 1500different compounds. It is also possible to assay multiple compounds ineach plate well. It is possible to assay several different plates perday; assay screens for up to about 6,000-20,000 different compounds ispossible using the integrated systems of the invention. More recently,microfluidic approaches to reagent manipulation have been developed.

The molecule of interest can be bound to the solid state component,directly or indirectly, via covalent or non-covalent linkage, e.g., viaa tag. The tag can be any of a variety of components. In general, amolecule which binds the tag (a tag binder) is fixed to a solid support,and the tagged molecule of interest (e.g., the taste transductionmolecule of interest) is attached to the solid support by interaction ofthe tag and the tag binder.

A number of tags and tag binders can be used, based upon known molecularinteractions well described in the literature. For example, where a taghas a natural binder, for example, biotin, protein A, or protein G, itcan be used in conjunction with appropriate tag binders (avidin,streptavidin, neutravidin, the Fc region of an immunoglobulin, etc.).Antibodies to molecules with natural binders such as biotin are alsowidely available and appropriate tag binders (see, SIGMA Immunochemicals1998 catalogue SIGMA, St. Louis Mo.).

Similarly, any haptenic or antigenic compound can be used in combinationwith an appropriate antibody to form a tag/tag binder pair. Thousands ofspecific antibodies are commercially available and many additionalantibodies are described in the literature. For example, in one commonconfiguration, the tag is a first antibody and the tag binder is asecond antibody which recognizes the first antibody. In addition toantibody-antigen interactions, receptor-ligand interactions are alsoappropriate as tag and tag-binder pairs. For example, agonists andantagonists of cell membrane receptors (e.g., cell receptor-ligandinteractions such as transferrin, c-kit, viral receptor ligands,cytokine receptors, chemokine receptors, interleukin receptors,immunoglobulin receptors and antibodies, the cadherein family, theintegrin family, the selectin family, and the like; see, e.g., Pigott &Power, The Adhesion Molecule Facts Book I (1993)). Similarly, toxins andvenoms, viral epitopes, hormones (e.g., opiates, steroids, etc.),intracellular receptors (e.g., which mediate the effects of varioussmall ligands, including steroids, thyroid hormone, retinoids andvitamin D; peptides), drugs, lectins, sugars, nucleic acids (both linearand cyclic polymer configurations), oligosaccharides, proteins,phospholipids and antibodies can all interact with various cellreceptors.

Synthetic polymers, such as polyurethanes, polyesters, polycarbonates,polyureas, polyamides, polyethyleneimines, polyarylene sulfides,polysiloxanes, polyimides, and polyacetates can also form an appropriatetag or tag binder. Many other tag/tag binder pairs are also useful inassay systems described herein, as would be apparent to one of skillupon review of this disclosure.

Common linkers such as peptides, polyethers, and the like can also serveas tags, and include polypeptide sequences, such as poly gly sequencesof between about 5 and 200 amino acids. Such flexible linkers are knownto persons of skill in the art. For example, poly(ethelyne glycol)linkers are available from Shearwater Polymers, Inc. Huntsville, Ala.These linkers optionally have amide linkages, sulfhydryl linkages, orheterofunctional linkages.

Tag binders are fixed to solid substrates using any of a variety ofmethods currently available. Solid substrates are commonly derivatizedor functionalized by exposing all or a portion of the substrate to achemical reagent which fixes a chemical group to the surface which isreactive with a portion of the tag binder. For example, groups which aresuitable for attachment to a longer chain portion would include amines,hydroxyl, thiol, and carboxyl groups. Aminoalkylsilanes andhydroxyalkylsilanes can be used to functionalize a variety of surfaces,such as glass surfaces. The constitutive of such solid phase biopolymerarrays is well described in the literature. See, e.g., Merrifield, J.Am. Chem. Soc., 85:2149-2154 (1963) (describing solid phase synthesisof, e.g., peptides); Geysen et al., J. Immun. Meth., 102:259-274 (1987)(describing synthesis of solid phase components on pins); Frank &Doring, Tetrahedron, 44:60316040 (1988) (describing synthesis of variouspeptide sequences on cellulose disks); Fodor et al., Science,251:767-777 (1991); Sheldon et al., Clinical Chemistry, 39(4):718-719(1993); and Kozal et al., Nature Medicine, 2(7):753759 (1996) (alldescribing arrays of biopolymers fixed to solid substrates).Non-chemical approaches for fixing tag binders to substrates includeother common methods, such as heat, cross-linking by UV radiation, andthe like.

3. Cell-based Assays

In a preferred embodiment of treatment, a combination of T1R proteins orpolypeptides are transiently or stably co-expressed in a eukaryotic celleither in unmodified forms or as chimeric, variant or truncatedreceptors with or preferably without a heterologous, chaperone sequencethat facilitates its maturation and targeting through the secretorypathway. Such T1R polypeptides can be expressed in any eukaryotic cell,such as HEK-293 cells. Preferably, the cells comprise a functional Gprotein, e.g., GαI5 or the chimeric G protein previously identified, oranother G protein that is capable of coupling the chimeric receptor toan intracellular signaling pathway or to a signaling protein such asphospholipase C. Also, preferably a cell will be produced that stablyco-expresses T1R1/T1R3 or T1R2/T1R3 as such cells have been found (asshown in the experimental examples) to exhibit enhanced responses totaste stimuli (relation to cells that transiently express the same T1Rcombination). Activation of T1R receptors in such cells can be detectedusing any standard method, such as by detecting changes in intracellularcalcium by detecting Fluo-4 dependent fluorescence in the cell. Such anassay is the basis of the experimental findings presented in thisapplication.

Activated GPCR receptors often are substrates for kinases thatphosphorylate the C-terminal tail of the receptor (and possibly othersites as well). Thus, activators will promote the transfer of ³²P fromradiolabeled ATP to the receptor, which can be assayed with ascintillation counter. The phosphorylation of the C-terminal tail willpromote the binding of arrestin-like proteins and will interfere withthe binding of G proteins. For a general review of GPCR signaltransduction and methods of assaying signal transduction, see, e.g.,Methods in Enzymology, vols. 237 and 238 (1994) and volume 96 (1983);Bourne et al., Nature, 10:349:117-27 (1991); Bourne et al., Nature,348:125-32 (1990); Pitcher et al., Annu. Rev. Biochem., 67:653-92(1998).

T1R modulation may be assayed by comparing the response of T1Rpolypeptides treated with a putative T1R modulator to the response of anuntreated control sample or a sample containing a known “positive”control. Such putative T1R modulators can include molecules that eitherinhibit or activate T1R polypeptide activity. In one embodiment, controlsamples (untreated with activators or inhibitors) are assigned arelative T1R activity value of 100. Inhibition of a T1R polypeptide isachieved when the T1R activity value relative to the control is about90%, optionally 50%, optionally 25-0%. Activation of a T1R polypeptideis achieved when the T1R activity value relative to the control is 110%,optionally 150%, 200-500%, or 1000-2000%.

Changes in ion flux may be assessed by determining changes in ionicpolarization (i.e., electrical potential) of the cell or membraneexpressing a T1R polypeptide. One means to determine changes in cellularpolarization is by measuring changes in current (thereby measuringchanges in polarization) with voltage-clamp and patch-clamp techniques(see, e.g., the “cell-attached” mode, the “inside-out” mode, and the“whole cell” mode, e.g., Ackerman et al., New Engl. J Med.,336:1575-1595 (1997)). Whole cell currents are conveniently determinedusing the standard. Other known assays include: radiolabeled ion fluxassays and fluorescence assays using voltage-sensitive dyes (see, e.g.,Vestergarrd-Bogind et al., J. Membrane Biol., 88:67-75 (1988); Gonzales& Tsien, Chem. Biol., 4:269-277 (1997); Daniel et al., J. Pharmacol.Meth., 25:185-193 (1991); Holevinsky et al., J. Membrane Biology,137:59-70 (1994)).

The effects of the test compounds upon the function of the polypeptidescan be measured by examining any of the parameters described above. Anysuitable physiological change that affects GPCR activity can be used toassess the influence of a test compound on the polypeptides of thisinvention. When the functional consequences are determined using intactcells or animals, one can also measure a variety of effects such astransmitter release, hormone release, transcriptional changes to bothknown and uncharacterized genetic markers (e.g., northern blots),changes in cell metabolism such as cell growth or pH changes, andchanges in intracellular second messengers such as Ca²⁺, IP3, cGMP, orcAMP.

Preferred assays for GPCRs include cells that are loaded with ion orvoltage sensitive dyes to report receptor activity. Assays fordetermining activity of such receptors can also use known agonists andantagonists for other G protein-coupled receptors as controls to assessactivity of tested compounds. In assays for identifying modulatorycompounds (e.g., agonists, antagonists), changes in the level of ions inthe cytoplasm or membrane voltage will be monitored using an ionsensitive or membrane voltage fluorescent indicator, respectively. Amongthe ion-sensitive indicators and voltage probes that may be employed arethose disclosed in the Molecular Probes 1997 Catalog. For Gprotein-coupled receptors, promiscuous G proteins such as Gα15 and Gα16can be used in the assay of choice (Wilkie et al., Proc. Nat'1 Acad.Sci., 88:10049-10053 (1991)).

Receptor activation initiates subsequent intracellular events, e.g.,increases in second messengers. Activation of some G protein-coupledreceptors stimulates the formation of inositol triphosphate (IP3)through phospholipase C-mediated hydrolysis of phosphatidylinositol(Berridge & Irvine, Nature, 312:315-21 (1984)). IP3 in turn stimulatesthe release of intracellular calcium ion stores. Thus, a change incytoplasmic calcium ion levels, or a change in second messenger levelssuch as IP3 can be used to assess G protein-coupled receptor function.Cells expressing such G protein-coupled receptors may exhibit increasedcytoplasmic calcium levels as a result of contribution from both calciumrelease from intracellular stores and extracellular calcium entry viaplasma membrane ion channels.

In a preferred embodiment, T1R polypeptide activity is measured bystably or transiently co-expressing T1R genes, preferably stably, in aheterologous cell with a promiscuous G protein that links the receptorto a phospholipase C signal transduction pathway (see Offermanns &Simon, J. Biol. Chem., 270:15175-15180 (1995)). In a preferredembodiment, the cell line is HEK-293 (which does not normally expressT1R genes) and the promiscuous G protein is Gα15 (Offermanns & Simon,supra). Modulation of taste transduction is assayed by measuring changesin intracellular Ca²⁺ levels, which change in response to modulation ofthe T1R signal transduction pathway via administration of a moleculethat associates with T1R polypeptides. Changes in Ca²⁺ levels areoptionally measured using fluorescent Ca²⁺ indicator dyes andfluorometric imaging.

In another embodiment, phosphatidyl inositol (PI) hydrolysis can beanalyzed according to U.S. Pat. No. 5,436,128, herein incorporated byreference. Briefly, the assay involves labeling of cells with3H-myoinositol for 48 or more hrs. The labeled cells are treated with atest compound for one hour. The treated cells are lysed and extracted inchloroform-methanol-water after which the inositol phosphates wereseparated by ion exchange chromatography and quantified by scintillationcounting. Fold stimulation is determined by calculating the ratio of cpmin the presence of agonist, to cpm in the presence of buffer control.Likewise, fold inhibition is determined by calculating the ratio of cpmin the presence of antagonist, to cpm in the presence of buffer control(which may or may not contain an agonist).

Other receptor assays can involve determining the level of intracellularcyclic nucleotides, e.g., cAMP or cGMP. In cases where activation of thereceptor results in a decrease in cyclic nucleotide levels, it may bepreferable to expose the cells to agents that increase intracellularcyclic nucleotide levels, e.g., forskolin, prior to adding areceptor-activating compound to the cells in the assay. In oneembodiment, the changes in intracellular cAMP or cGMP can be measuredusing immunoassays. The method described in Offermanns & Simon, J. Bio.Chem., 270:15175-15180 (1995), may be used to determine the level ofcAMP. Also, the method described in Felley-Bosco et al., Am. J. Resp.Cell and Mol. Biol., 11:159-164 (1994), may be used to determine thelevel of cGMP. Further, an assay kit for measuring cAMP and/or cGMP isdescribed in U.S. Pat. No. 4,115,538, herein incorporated by reference.

In another embodiment, transcription levels can be measured to assessthe effects of a test compound on signal transduction. A host cellcontaining T1R polypeptides of interest is contacted with a testcompound for a sufficient time to effect any interactions, and then thelevel of gene expression is measured. The amount of time to effect suchinteractions may be empirically determined, such as by running a timecourse and measuring the level of transcription as a function of time.The amount of transcription may be measured by using any method known tothose of skill in the art to be suitable. For example, mRNA expressionof the protein of interest may be detected using northern blots or theirpolypeptide products may be identified using immunoassays.Alternatively, transcription based assays using reporter gene may beused as described in U.S. Pat. No. 5,436,128, herein incorporated byreference. The reporter genes can be, e.g., chloramphenicolacetyltransferase, luciferase, beta-galactosidase beta-lactamase andalkaline phosphatase. Furthermore, the protein of interest can be usedas an indirect reporter via attachment to a second reporter such asgreen fluorescent protein (see, e.g., Mistili & Spector, NatureBiotechnology, 15:961-964 (1997)).

The amount of transcription is then compared to the amount oftranscription in either the same cell in the absence of the testcompound, or it may be compared with the amount of transcription in asubstantially identical cell that lacks the T1R polypeptide(s) ofinterest. A substantially identical cell may be derived from the samecells from which the recombinant cell was prepared but which had notbeen modified by introduction of heterologous DNA. Any difference in theamount of transcription indicates that the test compound has in somemanner altered the activity of the T1R polypeptides of interest.

4. Transgenic Non-human Animals Expressing Chemosensory Receptors

Non-human animals expressing a combination of T1R taste receptorsequences of the invention can also be used for receptor assays. Suchexpression can be used to determine whether a test compound specificallybinds to a mammalian taste transmembrane receptor complex in vivo bycontacting a non-human animal stably or transiently transfected withnucleic acids encoding chemosensory receptors or ligand-binding regionsthereof with a test compound and determining whether the animal reactsto the test compound by specifically binding to the receptor polypeptidecomplex.

Animals transfected or infected with the vectors of the invention areparticularly useful for assays to identify and characterize tastestimuli that can bind to a specific or sets of receptors. Suchvector-infected animals expressing human taste receptor sequences can beused for in vivo screening of taste stimuli and their effect on, e.g.,cell physiology (e.g., on taste neurons), on the CNS, or behavior.Alternatively, stable cell lines that express a T1R or combinationthereof, can be used as nucleic transfer donors to produced clonedtransgenic animals that stably express a particular T1R or combination.Methods of using nucleic transfer to produce cloned animals that expressa desired heterologous DNA are the subject of several issued U.S.patents granted to the University of Massachusetts (licensed to AdvancedCell Technology, Inc.) and Roslin Institute (licensed to Geron Corp.).

Means to infect/express the nucleic acids and vectors, eitherindividually or as libraries, are well known in the art. A variety ofindividual cell, organ, or whole animal parameters can be measured by avariety of means. The T1R sequences of the invention can be for exampleco-expressed in animal taste tissues by delivery with an infectingagent, e.g., adenovirus expression vector.

The endogenous taste receptor genes can remain functional and wild-type(native) activity can still be present. In other situations, where it isdesirable that all taste receptor activity is by the introducedexogenous hybrid receptor, use of a knockout line is preferred. Methodsfor the constitutive of non-human transgenic animals, particularlytransgenic mice, and the selection and preparation of recombinantconstructs for generating transformed cells are well known in the art.

Constitutive of a “knockout” cell and animal is based on the premisethat the level of expression of a particular gene in a mammalian cellcan be decreased or completely abrogated by introducing into the genomea new DNA sequence that serves to interrupt some portion of the DNAsequence of the gene to be suppressed. Also, “gene trap insertion” canbe used to disrupt a host gene, and mouse embryonic stem (ES) cells canbe used to produce knockout transgenic animals (see, e.g., Holzschu,Transgenic Res 6:97-106 (1997)). The insertion of the exogenous istypically by homologous recombination between complementary nucleic acidsequences. The exogenous sequence is some portion of the target gene tobe modified, such as exonic, intronic or transcriptional regulatorysequences, or any genomic sequence which is able to affect the level ofthe target gene's expression; or a combination thereof. Gene targetingvia homologous recombination in pluripotential embryonic stem cellsallows one to modify precisely the genomic sequence of interest. Anytechnique can be used to create, screen for, propagate, a knockoutanimal, e.g., see Bijvoet, Hum. Mol. Genet. 7:53-62 (1998); Moreadith,J. Mol. Med. 75:208-216 (1997); Tojo, Cytotechnology 19:161-165 (1995);Mudgett, Methods Mol. Biol. 48:167-184 (1995); Longo, Transgenic Res.6:321-328 (1997); U.S. Pat. Nos. 5,616,491; 5,464,764; 5,631,153;5,487,992; 5,627,059; 5,272,071; WO 91/09955; WO93/09222; WO 96/29411;WO 95/31560; WO 91/12650.

The nucleic acids of the invention can also be used as reagents toproduce “knockout” human cells and their progeny. Likewise, the nucleicacids of the invention can also be used as reagents to produce“knock-ins” in mice. The human or rat T1R gene sequences can replace theorthologous T1R in the mouse genome. In this way, a mouse expressing ahuman or rat T1R is produced. This mouse can then be used to analyze thefunction of human or rat T1Rs, and to identify ligands for such T1Rs.

a. Modulators

The compounds tested as modulators of a T1R family member can be anysmall chemical compound, or a biological entity, such as a protein,nucleic acid or lipid. Examples thereof include 5¹ IMP and 5¹ GMP.Essentially any chemical compound can be used as a potential modulatoror ligand in the assays of the invention, although most often compoundsthat are soluble in aqueous solutions are tested. Assays can be designedto screen large chemical libraries by automating the assay steps andproviding compounds from any convenient source; these assays aretypically run in parallel (e.g., in microtiter formats on microtiterplates in robotic assays). It will be appreciated that chemicallibraries can be synthesized by one of many chemical reactions (e.g.Senomyx proprietary chemistries). Additionally, there are many suppliersof chemical compounds, including Sigma (St. Louis, Mo.), Aldrich (St.Louis, Mo.), Sigma-Aldrich (St. Louis, Mo.), Fluka Chemika-BiochemicaAnalytika (Buchs, Switzerland) and the like.

In one preferred embodiment, high throughput screening methods involveproviding a combinatorial chemical or peptide library containing a largenumber of potential taste affecting compounds (potential modulator orligand compounds). Such “combinatorial chemical libraries” or “ligandlibraries” are then screened in one or more assays, as described herein,to identify those library members (particular chemical species orsubclasses) that display a desired characteristic activity. Thecompounds thus identified can serve as conventional “lead compounds” orcan themselves be used as potential or actual taste modulators.

Preferably, such libraries will be screened against cells or cell linesthat stably express a T1R or combination of T1Rs, i.e. T1R1/T1R3 orT1R2/T1R3 and preferably a suitable G protein, e.g. G_(α15). As shown inthe examples infra, such stable cell lines exhibit very pronouncedresponses to taste stimuli, e.g. umami or sweet taste stimuli. However,cells and cell lines that transiently express one or more T1Rs may alsobe used in such assays.

A combinatorial chemical library is a collection of diverse chemicalcompounds generated by either chemical synthesis or biologicalsynthesis, by combining a number of chemical “building blocks” such asreagents. For example, a linear combinatorial chemical library such as apolypeptide library is formed by combining a set of chemical buildingblocks (amino acids) in every possible way for a given compound length(i.e., the number of amino acids in a polypeptide compound). Thousandsto millions of chemical compounds can be synthesized through suchcombinatorial mixing of chemical building blocks.

Preparation and screening of combinatorial chemical libraries is wellknown to those of skill in the art. Such combinatorial chemicallibraries include, but are not limited to, peptide libraries (see, e.g.,U.S. Pat. No. 5,010,175, Furka, Int. J. Pept. Prot. Res., 37:487-493(1991) and Houghton et al., Nature, 354:84-88 (1991)). Other chemistriesfor generating chemical diversity libraries can also be used. Suchchemistries include, but are not limited to: peptoids (e.g., PCTPublication No. WO 91/19735), encoded peptides (e.g., PCT Publication WO93/20242), random bio-oligomers (e.g., PCT Publication No. WO 92/00091),benzodiazepines (e.g., U.S. Pat. No. 5,288,514), diversomers such ashydantoins, benzodiazepines and dipeptides (Hobbs et al., Proc. Nat.Acad. Sci., 90:6909-6913 (1993)), vinylogous polypeptides (Hagihara etal., J. Amer. Chem. Soc., 114:6568 (1992)), nonpeptidal peptidomimeticswith glucose scaffolding (Hirschmann et al., J. Amer. Chem. Soc.,114:9217-9218 (1992)), analogous organic syntheses of small compoundlibraries (Chen et al., J. Amer. Chem. Soc., 116:2661 (1994)),oligocarbamates (Cho et al., Science, 261:1303 (1993)), peptidylphosphonates (Campbell et al., J. Org. Chem., 59:658 (1994)), nucleicacid libraries (Ausubel, Berger and Sambrook, all supra), peptidenucleic acid libraries (U.S. Pat. No. 5,539,083), antibody libraries(Vaughn et al., Nature Biotechnology, 14(3):309-314 (1996) andPCT/US96/10287), carbohydrate libraries (Liang et al., Science,274:1520-1522 (1996) and U.S. Pat. No. 5,593,853), small organicmolecule libraries (benzodiazepines, Baum, C&EN, January 18, page 33(1993); thiazolidinones and metathiazanones, U.S. Pat. No. 5,549,974;pynrolidines, U.S. Pat. Nos. 5,525,735 and 5,519,134; morpholinocompounds, U.S. Pat. No. 5,506,337; benzodiazepines, U.S. Pat. No.5,288,514, and the like).

Devices for the preparation of combinatorial libraries are commerciallyavailable (see, e.g., 357 MPS, 390 MPS (Advanced Chem Tech, LouisvilleKy.), Symphony (Rainin, Woburn, Mass.), 433A (Applied Biosystems, FosterCity, Calif.), 9050 Plus (Millipore, Bedford, Mass.)). In addition,numerous combinatorial libraries are themselves commercially available(see, e.g., ComGenex, Princeton, N.J.; Tripos, Inc., St. Louis, Mo.; 3DPharmaceuticals, Exton, Pa.; Martek Biosciences; Columbia, Md.; etc.).

In one aspect of the invention, the T1R modulators can be used in anyfood product, confectionery, pharmaceutical composition, or ingredientthereof to thereby modulate the taste of the product, composition, oringredient in a desired manner. For instance, T1R modulators thatenhance sweet taste sensation can be added to sweeten a product orcomposition; T1R modulators that enhance umami taste sensation can beadded to foods to increase savory tastes. Alternatively, T1R antagonistscan be used to block sweet and/or umami taste.

b. Kits

T1R genes and their homologs are useful tools for identifyingchemosensory receptor cells, for forensics and paternity determinations,and for examining taste transduction. T1R family member-specificreagents that specifically hybridize to T1R nucleic acids, such as T1Rprobes and primers, and T1R specific reagents that specifically bind toa T1R polypeptide, e.g., T1R antibodies are used to examine taste cellexpression and taste transduction regulation.

Nucleic acid assays for the presence of DNA and RNA for a T1R familymember in a sample include numerous techniques are known to thoseskilled in the art, such as southern analysis, northern analysis, dotblots, RNase protection, S1 analysis, amplification techniques such asPCR, and in situ hybridization. In in situ hybridization, for example,the target nucleic acid is liberated from its cellular surroundings insuch as to be available for hybridization within the cell whilepreserving the cellular morphology for subsequent interpretation andanalysis. The following articles provide an overview of the art of insitu hybridization: Singer et al., Biotechniques, 4:230250 (1986); Haaseet al., Methods in Virology, vol. VII, pp. 189-226 (1984); and NucleicAcid Hybridization: A Practical Approach (Names et al., eds. 1987). Inaddition, a T1R polypeptide can be detected with the various immunoassaytechniques described above. The test sample is typically compared toboth a positive control (e.g., a sample expressing a recombinant T1Rpolypeptide) and a negative control.

The present invention also provides for kits for screening formodulators of T1R family members. Such kits can be prepared from readilyavailable materials and reagents. For example, such kits can compriseany one or more of the following materials: T1R nucleic acids orproteins, reaction tubes, and instructions for testing T1R activity.Optionally, the kit contains a biologically active T1R receptor or cellline that stably or transiently expresses a biologically active T1Rcontaining taste receptor. A wide variety of kits and components can beprepared according to the present invention, depending upon the intendeduser of the kit and the particular needs of the user.

EXAMPLES

While the invention has been described in detail supra, the followingexamples are provided to illustrate preferred embodiments. Theseexamples are intended to be illustrative and not limitative of the scopeof the invention.

In the protein sequences presented herein, the one-letter code X or Xaarefers to any of the twenty common amino acid residues. In the DNAsequences presented herein, the one letter codes N or n refers to any ofthe of the four common nucleotide bases, A, T, C, or G.

Example 1 Production of Intronless hT1R Expression Constructs

Intronless hT1R expression constructs were cloned by a combination ofcDNA-based and genomic DNA-based methods. To generate the full-lengthhT1R1 expression construct, two 5′ coding exons identified in a clonedhT1R1 interval (accession # AL159177) were combined by PCR-overlap, andthen joined to a 5′-truncated testis cDNA clone. The hT1R2 expressionconstruct was generated from a partially sequenced hT1R2 genomicinterval. Two missing hT1R2 5′ exons were identified by screeningshotgun libraries of the cloned genomic interval using probes derivedfrom the corresponding rat coding sequence. Coding exons were thencombined by PCR-overlap to produce the full-length expression construct.The hT1R3 expression construct was generated by PCR-overlap from asequenced hT1R3 genomic interval (accession # AL139287). Rat T1R3 wasisolated from a rat taste tissue-derived cDNA library using an rT1R3exon fragment generated by hT1R3-based degenerate PCR. The partial hT1R1cDNA, rT1R2 cDNA, and partial hT1R2 genomic sequences were obtained fromDr. Charles Zuker (University of California, San Diego).

The nucleic acid and amino acid sequences for the above-identified T1Rcloned sequences as well as other full-length and partial T1R sequencesare set forth below:

Amino Acid Sequence rT1R3 SEQ ID NO: 4MPGLAILGLSLAAFLELGMGSSLCLSQQFKAQGDYILGGLFPLGTTEEATLNQRTQPNGILCTRFSPLGLFLAMAMKMAVEEINNGSALLPGLRLGYDLFDTCSEPVVTMKPSLMFMAKVGSQSIAAYCNYTQYQPRVLAVIGPHSSELALITGKFFSFFLMPQVSYSASMDRLSDRETFPSFFRTVPSDRVQLQAVVTLLQNFSWNWVAALGSDDDYGREGLSIFSGLANSRGICIAHEGLVPQHDTSGQQLGKVVDVLRQVNQSKVQVVVLFASARAVYSLFSYSILHDLSPKVWVASESWLTSDLVMTLPNIARVGTVLGFLQRGALLPEFSHYVETRLALAADPTFCASLKAELDLEERVMGPRCSQCDYIMLQNLSSGLMQNLSAGQLHHQIFATYAAVYSVAQALHNTLQCNVSHCHTSEPVQPWQLLENMYNMSFRARDLTLQFDAKGSVDMEYDLKMWVWQSPTPVLHTVGTFNGTLQLQHSKMYWPGNQVPVSQCSRQCKDGQVRRVKGFHSCCYDCVDCKAGSYRKHPDDFTCTPCGKDQWSPEKSTTCLPRRPKFLAWGEPAVLSLLLLLCLVLGLTLAALGLFVHYWDSPLVQASGGSLFCFGLICLGLFCLSVLLFPGRPRSASCLAQQPMAHLPLTGCLSTLFLQAAEIFVESELPLSWANWLCSYLRGPWAWLVVLLATLVEAALCAWYLMAFPPEVVTDWQVLPTEVLEHCRMRSWVSLGLVHITNAVLAFLCFLGTFLVQSQPGRYNRARGLTFAMLAYFIIWVSFVPLLANVQVAYQPAVQMGAILFCALGILATFHLPKCYVLLWLPELNTQEFFLGRSPKEASDGNSGSSEATRGHSE Amino Acid Sequence hT1R1 SEQ ID NO: 5MLLCTARLVGLQLLISCCWAFACHSTESSPDFTLPGDYLLAGLFPLHSGCLQVRHRPEVTLCDRSCSFNEHGYHLFQAMRLGVEEINNSTALLPNITLGYQLYDVCSDSANVYATLRVLSLPGQHHIELQGDLLHYSPTVLAVIGPDSTNRAATTAALLSPFLVPMISYAASSETLSVKRQYPSFLRTIPNDKYQVETMVLLLQKFGWTWISLVGSSDDYGQLGVQALENQATGQGICIAFKDIMPFSAQVGDERMQCLMRHLAQAGATVVVVFSSRQLARVFFESVVLTNLTGKVWVASEAWALSRHITGVPGIQRIGMVLGVAIQKRAVPGLKAFEEAYARADKKAPRPCHKGSWCSSNQLCRECQAFMAHTMPKLKAFSMSSAYNAYRAVYAVAHGLHQLLGCASGACSRGRVYPWQLLEQIHKVHFLLHKDTVAFNDNRDPLSSYNIIAWDWNGPKWTFTVLGSSTWSPVQLNINETKIQWHGKDNQVPKSVCSSDCLEGHQRVVTGFHHCCFECVPCGAGTFLNKSDLYRCQPCGKEEWAPEGSQTCFPRTVVFLALREHTSWVLLAANTLLLLLLLGTAGLFAWHLDTPVVRSAGGRLCFLMLGSLAAGSGSLYGFFGEPTRPACLLRQALFALGFTIFLSCLTVRSFQLIIIFKFSTKVPTFYHAWVQNHGAGLFVMISSAAQLLICLTWLVVWTPLPAREYQRFPHLVMLECTETNSLGFILAFLYNGLLSISAFACSYLGKDLPENYNEAKCVTFSLLFNFVSWIAFFTTASVYDGKYLPAANMMAGLSSLSSGEGGYFLPKCYVILCRPDLNSTEHFQASIQDYTRRCGS TAmino Acid Sequence hT1R2 SEQ ID NO: 6MGPRAKTICSLFFLLWVLAEPAENSDFYLPGDYLLGGLFSLHANMKGIVHLNFLQVPMCKEYEVKVIGYNLMQAMRFAVEEINNDSSLLPGVLLGYEIVDVCYISNNVQPVLYFLAHEDNLLPIQEDYSNYISRVVAVIGPDNSESVMTVANFLSLFLLPQITYSAISDELRDKVRFPALLRTTPSADHHVEAMVQLMLHFRWNWIIVLVSSDTYGRDNGQLLGERVARRDICIAFQETLPTLQPNQNMTSEERQRLVTIVDKLQQSTARVVVVFSPDLTLYHFFNEVLRQNFTGAVWIASESWAIDPVLHNLTELGHLGTFLGITIQSVPIPGFSEFREWGPQAGPPPLSRTSQSYTCNQECDNCLNATLSFNTILRLSGERVVYSVYSAVYAVAHALHSLLGCDKSTCTKRVVYPWQLLEEIWKVNFTLLDHQIFFDPQGDVALHLEIVQWQWDRSQNPFQSVASYYPLQRQLKNIQDISWHTVNNTIPMSMCSKRCQSGQKKKPVGIHVCCFECIDCLPGTFLNHTEDEYECQACPNNEWSYQSETSCFKRQLVFLEWHEAPTIAVALLAALGFLSTLAILVIFWRHFQTPIVRSAGGPMCFLMLTLLLVAYMVVPVYVGPPKVSTCLCRQALFPLCFTICISCIAVRSFQIVCAFKMASRFPRAYSYWVRYQGPYVSMAFITVLKMVIVVIGMLATGLSPTTRTDPDDPKITIVSCNPNYRNSLLFNTSLDLLLSVVGFSFAYMGKELPTNYNEAKFITLSMTFYFTSSVSLCTFMSAYSGVLVTIVDLLVTVLNLLAISLGYFGPKCYMILFYPERNTPAYFNSMIQGYTMRRDAmino Acid Sequence hT1R3 SEQ ID NO: 7MLGPAVLGLSLWALLHPGTGAPLCLSQQLRMKGDYVLGGLFPLGEAEEAGLRSRTRPSSPVCTRFSSNGLLWALAMKMAVEEINNKSDLLPGLRLGYDLFDTCSFPVVAMKPSLMFLAKAGSRDIAAYCNYTQYQPRVLAVIGPHSSELAMVTGKFFSFFLMPQVSYGASMELLSARETFPSFFRTVPSDRVQLTAAAELLQEFGWNWVAALGSDDEYGRQGLSIFSALAAARGICIAHEGLVPLPRADDSRLGKVODVLHQVNQSSVQVVLLFASVHAAHALFNYSISSRLSPKVWVASEAWLTSDLVMGLPGMAQMGTVLGFLQRGAQLHEFPQYVKTHLALATDPAFCSALGEREQGLEEDVVGQRCPQCDCITLQNVSAGLNHHQTFSVYAAVYSVAQALHNTLQCNASGCPAQDPVKPWQLLENMYNLTFHVGGLPLRFDSSGNVDMEYDLKLWVWQGSVPRLHDVGRFNGSLRTERLKIRWHTSDNQKPVSRCSRQCQEGQVRRVKGFHSCCYDCVDCEAGSYRQNPDDIACTFCGQDEWSPERSTRCFRRRSRFLAWGEPAVLLLLLLLSLALGLVLAALGLFVHHRDSPLVQASGGPLACFGLVCLGLVCLSVLLFPGQPSPARCLAQQPLSHLPLTGCLSTLFLQAAEIFVESELPLSWADRLSGCLRGPWAWLVVLLAMLVEVALCTWYLVAFPPEVVTDWHMLPTEALVHCRTRSWVSFGLAHATNATLAFLCFLGTFLVRSQPGRYNRARGLTFAMLAYFITWVSFVPLLANVQVVLRPAVQMGALLLCVLGILAAFHLPRCYLLMRQPGLNTPEFFLGGGPGDAQGQNDGNTGNQGKHE Nucleic Acid Sequence hT1R1 SEQ ID NO: 8ATGCTGCTCTGCACGGCTCGCCTGGTCGGCCTGCAGCTTCTCATTTCCTGCTGCTGGGCCTTTGCCTGCCATAGCACGGAGTCTTCTCCTGACTTCACCCTCCCCGGAGATTACCTCCTGGCAGGCCTGTTCCCTCTCCATTCTGGCTGTCTGCAGGTGAGGCACAGACCCGAGGTGACCCTGTGTGACAGGTCTTGTAGCTTCAATGAGCATGGCTACCACCTCTTCCAGGCTATGCGGCTTGGGGTTGAGGAGATAAACAACTCCACGGCCCTGCTGCCCAACATCACCCTGGGGTACCAGCTGTATGATGTGTGTTCTGACTCTGCCAATGTGTATGCCACGCTGAGAGTGCTCTCCCTGCCAGGGCAACACCACATAGAGCTCCAAGGAGACCTTCTCCACTATTCCCCTACGGTGCTGGCAGTGATTGGGCCTGACAGCACCAACCGTGCTGCCACCACAGCCGCCCTGCTGAGCCCTTTCCTGGTGCCCATGATTAGCTATGCGGCCAGCAGCGAGACGCTCAGCGTGAAGCGGCAGTATCCCTCTTTCCTGCGCACCATCCCCAATGACAAGTACCAGGTGGAGACCATGGTGCTGCTGCTGCAGAAGTTCGGGTGGACCTGGATCTCTCTGGTTGGCAGCAGTGACGACTATGGGCAGCTAGGGGTGCAGGCACTGGAGAACCAGGCCACTGGTCAGGGGATCTGCATTGCTTTCAAGGACATCATGCCCTTCTCTGCCCAGGTGGGCGATGAGAGGATGCAGTGCCTCATGCGCCACCTGGCCCAGGCCGGGGCCACCGTCGTGGTTGTTTTTTCCAGCCGGCAGTTGGCCAGGGTGTTTTTCGAGTCCGTGGTGCTGACCAACCTGACTGGCAAGGTGTGGGTCGCCTCAGAAGCCTGGGCCCTCTCCAGGCACATCACTGGGGTGCCCGGGATCCAGCGCATTGGGATGGTGCTGGGCGTGGCCATCCAGAAGAGGGCTGTCCCTGGCCTGAAGGCGTTTGAAGAAGCCTATGCCCGGGCAGACAAGAAGGCCCCTAGGCCTTGCCACAAGGGCTCCTGGTGCAGCAGCAATCAGCTCTGCAGAGAATGCCAAGCTTTCATGGCACACACGATGCCCAAGCTCAAAGCCTTCTCCATGAGTTCTGCCTACAACGCATACCGGGCTGTGTATGCGGTGGCCCATGGCCTCCACCAGCTCCTGGGCTGTGCCTCTGGAGCTTGTTCCAGGGGCCGAGTCTACCCCTGGCAGCTTTTGGAGCAGATCCACAAGGTGCATTTCCTTCTACACAAGGACACTGTGGCGTTTAATGACAACAGAGATCCCCTCAGTAGCTATAACATAATTGCCTGGGACTGGAATGGACCCAAGTGGACCTTCACGGTCCTCGGTTCCTCCACATGGTCTCCAGTTCAGCTAAACATAAATGAGACCAAAATCCAGTGGCACGGAAAGGACAACCAGGTGCCTAAGTCTGTGTGTTCCAGCGACTGTCTTGAAGGGCACCAGCGAGTGGTTACGGGTTTCCATCACTGCTGCTTTGAGTGTGTGCCCTGTGGGGCTGGGACCTTCCTCAACAAGAGTGACCTCTACAGATGCCAGCCTTGTGGGAAAGAAGAGTGGGCACCTGAGGGAAGCCAGACCTGCTTCCCGCGCACTGTGGTGTTTTTGGCTTTGCGTGAGCACACCTCTTGGGTGCTGCTGGCAGCTAACACGCTGCTGCTGCTGCTGCTGCTTGGGACTGCTGGCCTGTTTGCCTGGCACCTAGACACCCCTGTGGTGAGGTCAGCAGGGGGCCGCCTGTGCTTTCTTATGCTGGGCTCCCTGGCAGCAGGTAGTGGCAGCCTCTATGGCTTCTTTGGGGAACCCACAAGGCCTGCGTGCTTGCTACGCCAGGCCCTCTTTGCCCTTGGTTTCACCATCTTCCTGTCCTGCCTGACAGTTCGCTCATTCCAACTAATCATCATCTTCAAGTTTTCCACCAAGGTACCTACATTCTACCACGCCTGGGTCCAAAACCACGGTGCTGGCCTGTTTGTGATGATCAGCTCAGCGGCCCAGCTGCTTATCTGTCTAACTTGGCTGGTGGTGTGGACCCCACTGCCTGCTAGGGAATACCAGCGCTTCCCCCATCTGGTGATGCTTGAGTGCACAGAGACCAACTCCCTGGGCTTCATACTGGCCTTCCTCTACAATGGCCTCCTCTCCATCAGTGCCTTTGCCTGCAGCTACCTGGGTAAGGACTTGCCAGAGAACTACAACGAGGCCAAATGTGTCACCTTCAGCCTGCTCTTCAACTTCGTGTCCTGGATCGCCTTCTTCACCACGGCCAGCGTCTACGACGGCAAGTACCTGCCTGCGGCCAACATGATGGCTGGGCTGAGCAGCCTGAGCAGCGGCTTCGGTGGGTATTTTCTGCCTAAGTGCTACGTGATCCTCTGCCGCCCAGACCTCAACAGCACAGAGCACTTCCAGGCCTCCATTCAGGACTACACGAGGCGCTGCGGCTCC ACCTGANucleic Acid Sequence hT1R3 SEQ ID NO 9ATGCTGGGCCCTGCTGTCCTGGGCCTCAGCCTCTGGGCTCTCCTGCACCCTGGGACGGGGGCCCCATTGTGCCTGTCACAGCAACTTAGGATGAAGGGGGACTACGTGCTGGGGGGGCTGTTCCCCCTGGGCGAGGCCGAGGAGGCTGGCCTCCGCAGCCGGACACGGCCCAGCAGCCCTGTGTGCACCAGGTTCTCCTCAAACGGCCTGCTCTGGGCACTGGCCATGAAAATGGCCGTGGAGGAGATCAACAACAAGTCGGATCTGCTGCCCGGGCTGCGCCTGGGCTACGACCTCTTTGATACGTGCTCGGAGCCTGTGGTGGCCATGAAGCCCAGCCTCATGTTCCTGGCCAAGGCAGGCAGCCGCGACATCGCCGCCTACTGCAACTACACGCAGTACCAGCCCCGTGTGCTGGCTGTCATCGGGCCCCACTCGTCAGAGCTCGCCATGGTCACCGGCAAGTTCTTCAGCTTCTTCCTCATGCCCCAggtcagCTACGGTGCTAGCATGGAGCTGCTGAGCGCCCGGGAGACCTTCCCCTCCTTCTTCCGCACCGTGCCCAGCGACCGTGTGCAGCTGACGGCCGCCGCGGAGCTGCTGCAGGAGTTCGGCTGGAACTGGGTGGCCGCCCTGGGCAGCGACGACGAGTACGGCCGGCAGGGCCTGAGCATCTTCTCGGCCCTGGCCGCGGCACGCGGCATCTGCATCGCGCACGAGGGCCTGGTGCCGCTGCCCCGTGCCGATGACTCGCGGCTGGGGAAGGTGCAGGACGTCCTGCACCAGGTGAACCAGAGCAGCGTGCAGGTGGTGCTGCTGTTCGCCTCCGTGCACGCCGCCCACGCCCTCTTCAACTACAGCATCAGCAGCAGGCTCTCGCCCAAGGTGTGGGTGGCCAGCGAGGCCTGGCTGACCTCTGACCTGGTCATGGGGCTGCCCGGCATGGCCCAGATGGGCACGGTGCTTGGCTTCCTCCAGAGGGGTGCCCAGCTGCACGAGTTCCCCCAGTACGTGAAGACGCACCTGGCCCTGGCCACCGACCCGGCCTTCTGCTCTGCCCTGGGCGAGAGGGAGCAGGGTCTGGAGGAGGACGTGGTGGGCCAGCGCTGCCCGCAGTGTGACTGCATCACGCTGCAGAACGTGAGCGCAGGGCTAAATCACCACCAGACGTTCTCTGTCTACGCAGCTGTGTATAGCGTGGCCCAGGCCCTGCACAACACTCTTCAGTGCAACGCCTCAGGCTGCCCCGCGCAGGACCCCGTGAAGCCCTGGCAGCTCCTGGAGAACATGTACAACCTGACCTTCCACGTGGGCGGGCTGCCGCTGCGGTTCGACAGCAGCGGAAACGTGGACATGGAGTACGACCTGAAGCTGTGGGTGTGGCAGGGCTCAGTGCCCAGGCTCCACGACGTGGGCAGGTTCAACGGCAGCCTCAGGACAGAGCGCCTGAAGATCCGCTGGCACACGTCTGACAACCAGAAGCCCGTGTCCCGGTGCTCGCGGCAGTGCCAGGAGGGCCAGGTGCGCCGGGTCAAGGGGTTCCACTCCTGCTGCTACGACTGTGTGGACTGCGAGGCGGGCAGCTACCGGCAAAACCCAGACGACATCGCCTGCACCTTTTGTGGCCAGGATGAGTGGTCCCCGGAGCGAAGCACACGCTGCTTCCGCCGCAGGTCTCGGTTCCTGGCATGGGGCGAGCCGGCTGTGCTGCTGCTGCTCCTGCTGCTGAGCCTGGCGCTGGGCCTTGTGCTGGCTGCTTTGGGGCTGTTCGTTCACCATCGGGACAGCCCACTGGTTCAGGCCTCGGGGGGGCCCCTGGCCTGCTTTGGCCTGGTGTGCCTGGGCCTGGTCTGCCTCAGCGTCCTCCTGTTCCCTGGCCAGCCCAGCCCTGCCCGATGCCTGGCCCAGCAGCCCTTGTCCCACCTCCCGCTCACGGGCTGCCTGAGCACACTCTTCCTGCAGGCGGCCGAGATCTTCGTGGAGTCAGAACTGCCTCTGAGCTGGGCAGACCGGCTGAGTGGCTGCCTGCGGGGGCCCTGGGCCTGGCTGGTGGTGCTGCTGGCCATGCTGGTGGAGGTCGCACTGTGCACCTGGTACCTGGTGGCCTTCCCGCCGGAGGTGGTGACGGACTGGCACATGCTGCCCACGGAGGCGCTGGTGCACTGCCGCACACGCTCCTGGGTCAGCTTCGGCCTAGCGCACGCCACCAATGCCACGCTGGCCTTTCTCTGCTTCCTGGGCACTTTCCTGGTGCGGAGCCAGCCGGGCTGCTACAACCGTGCCCGTGGCCTCACCTTTGCCATGCTGGCCTACTTCATCACCTGGGTCTCCTTTGTGCCCCTCCTGGCCAATGTGCAGGTGGTCCTCAGGCCCGCCGTGCAGATGGGCGCCCTCCTGCTCTGTGTCCTGGGCATCCTGGCTGCCTTCCACCTGCCCAGGTGTTACCTGCTCATGCGGCAGCCAGGGCTCAACACCCCCGAGTTCTTCCTGGGAGGGGGCCCTGGGGATGCCCAAGGCCAGAATGACGGGAACACAGGAAATCAGGGGAAACATGAGTGA Nucleic Acid Sequence hT1R2SEQ ID NO: 10ATGGGGCCCAGGGCAAAGACCATCTGCTCCCTGTTCTTCCTCCTATCGGTCCTGGCTGAGCCGGCTGAGAACTCGGACTTCTACCTGCCTGGGGATTACCTCCTGGGTGGCCTCTTCTCCCTCCATGCCAACATGAAGGGCATTGTTCACCTTAACTTCCTGCAGGTGCCCATGTGCAAGGAGTATGAAGTGAAGGTGATAGGCTACAACCTCATGCAGGCCATGCGCTTCGCGGTGGAGGAGATCAACAATGACAGCAGCCTGCTGCCTGGTGTGCTGCTGGGCTATGAGATCGTGGATGTGTGCTACATCTCCAACAATGTCCAGCCGGTGCTCTACTTCCTGGCACACGAGGACAACCTCCTTCCCATCCAAGAGGACTACAGTAACTACATTTCCCGTGTGGTGGCTGTCATTGGCCCTGACAACTCCGAGTCTGTCATGACTGTGGCCAACTTCCTCTCCCTATTTCTCCTTCCACAGATCACCTACAGCGCCATCAGCGATGAGCTGCGAGACAAGGTGCGCTTCCCGGCTTTGCTGCGTACCACACCCAGCGCCGACCACCACGTCGAGGCCATGGTGCAGCTGATGCTGCACTTCCGCTGGAACTGGATCATTGTGCTGGTGAGCAGCGACACCTATGGCCGCGACAATGGCAGCTGCTTGGCGAGCGCGTGGCCCGGCGCGACATCTGCATCGCCTTCCAGGAGACGCTGCCCACACTGCAGCCCAACCAGAACATGACGTCAGAGGAGCGCCAGCGCCTGGTGACCATTGTGGACAAGCTGCAGCAGAGCACAGCGCGCGTCGTGGTCGTGTTCTCGCCCGACCTGACCCTGTACCACTTCTTCAATGAGGTGCTGCGCCAGAACTTCACGGGCGCCGTGTGGATCGCCTCCGAGTCCTGGGCCATCGACCCGGTCCTGCACAACCTCACGGAGCTGGGCCACTTGGGCACCTTCCTGGGCATCACCATCCAGAGCGTGCCCATCCCGGGCTTCAGTGAGTTCCGCGAGTGGGGCCCACAGGCTGGGCCGCCACCCCTCAGCAGGACCAGCCAGAGCTATACCTGCAACCAGGAGTGCGACAACTGCCTGAACGCCACCTTGTCCTTCAACACCATTCTCAGGCTCTCTGGGGAGCGTGTCGTCTACAGCGTGTACTCTGCGGTCTATGCTGTGGCCCATGCCCTGCACAGCCTCCTCGGCTGTGACAAAAGCACCTGCACCAAGAGGGTGGTCTACCCCTGGCAGCTGCTTGAGGAGATCTGGAAGGTCAACTTCACTCTCCTGGACCACCAAATCTTCTTCGACCCGCAAGGGGACGTGGCTCTGCACTTGGAGATTGTCCAGTGGCAATGGGACCGGAGCCAGAATCCCTTCCAGAGCGTCGCCTCCTACTACCCCCTGCAGCGACAGCTGAAGAACATCCAAGACATCTCCTGGCACACCGTCAACAACACGATCCCTATGTCCATGTGTTCCAAGAGGTGCCAGTCAGGGCAAAAGAAGAAGCCTGTGGGCATCCACGTCTGCTGCTTCGAGTGCATCGACTGCCTTCCCGGCACCTTCCTCAACCACACTGAAGATGAATATGAATGCCAGGCCTGCCCGAATAACGAGTGGTCCTACCAGAGTGAGACCTCCTGOTTCAAGCGGCAGCTGGTCTTCCTGGAATGGCATGAGGCACCCACCATCGCTGTGGCCCTGCTGGCCGCCCTGGGCTTCCTCAGCACCCTGGCCATCCTGGTGATATTCTGGAGGCACTTCCAGACACCCATAGTTCGCTCGGCTGGGGGCCCCATGTGCTTCCTGATGCTGACACTGCTGCTGGTGGCATACATGGTGGTCCCGGTGTACGTGGGGCCGCCCAAGGTCTCCACCTGCCTCTGCCGCCAGGCCCTCTTTCCCCTCTGCTTCACAATTTGCATCTCCTGTATCGCCGTGCGTTCTTTCCAGATCGTCTGCGCCTTCAAGATGGCCAGCCGCTTCCCACGCGCCTACAGCTACTGGGTCCGCTACCAGGGGCCCTACGTCTCTATGGCATTTATCACGGTACTCAAAATGGTCATTGTGGTAATTGGCATGCTGGCCACGGGCCTCAGTCCCACCACCCGTACTGACCCCGATGACCCCAAGATCACAATTGTCTCCTGTAACCCCAACTACCGCAACAGCCTGCTGTTCAACACCAGCCTGGACCTGCTGCTCTCAGTGGTGGGTTTCAGCTTCGCCTACATGGGCAAAGAGCTGCCCACCAACTACAACGAGGCCAAGTTCATCACCCTCAGCATGACCTTCTATTTCACCTCATCCGTCTCCCTCTGCACCTTCATGTCTGCCTACAGCGGGGTGCTGGTCACCATCGTGGACCTCTTGGTCACTGTGCTCAACCTCCTGGCCATCAGCCTGGGCTACTTCGGCCCCAAGTGCTACATGATCCTCTTCTACCCGGAGCGCAACACGCCCGCCTACTTCAACAGCATGATCCAGGGCTACACCATGAGGAGGGACTAGNucleic Acid Sequence rT1R3 SEQ ID NO 11ATGCCGGGTTTGGCTATCTTGGGCCTCAGTCTGGCTGCTTTCCTGGAGCTTGGGATGGGGTCCTCTTTGTGTCTGTCACAGCAATTCAAGGCACAAGGGGACTATATATTGGGTGGACTATTTCCCCTGGGCACAACTGAGGAGGCCACTCTCAACCAGAGAACACAGCCCAACGGCATCCTATGTACCAGGTTCTCGCCCCTTGGTTTGTTCCTGGCCATGGCTATGAAGATGGCTGTAGAGGAGATCAACAATGGATCTGCCTTGCTCCCTGGGCTGCGACTGGGCTATGACCTGTTTGACACATGCTCAGAGCCAGTGGTCACCATGAAGCCCAGCCTCATGTTCATGGCCAAGGTGGGAAGTCAAAGCATTGCTGCCTACTGCAACTACACACAGTACCAACCCCGTGTGCTGGCTGTCATTGGTCCCCACTCATCAGAGCTTGCCCTCATTACAGGCAAGTTCTTCAGCTTCTTCCTCATGCCACAGGTCAGCTATAGTGCCAGCATGGATCGGCTAAGTGACCGGGAAACATTTCCATCCTTCTTCCGCACAGTGCCCAGTGACCGGGTGCAGCTGCAGGCCGTTGTGACACTGTTGCAGAATTTCAGCTGGAACTGGGTGGCTGCCTTAGGTAGTGATGATGACTATGGCCGGGAAGGTCTGAGCATCTTTTCTGGTCTGGCCAACTCACGAGGTATCTGCATTGCACACGAGGGCCTGGTGCCACAACATGACACTAGTGGCCAACAATTGGGCAAGGTGGTGGATGTGCTACGCCAAGTGAACCAAAGCAAAGTACAGGTGGTGGTGCTGTTTGCATCTGCCCGTGCTGTCTACTCCCTTTTTAGCTACAGCATCCTTCATGACCTCTCACCCAAGGTATGGGTGGCCAGTGAGTCCTGGCTGACCTCTGACCTGGTCATGACACTTCCCAATATTGCCCGTGTGGGCACTGTTCTTGGGTTTCTGCAGCGCGGTGCCCTACTGCCTGAATTTTCCCATTATGTGGAGACTCGCCTTGCCCTAGCTGCTGACCCAACATTCTGTGCCTCCCTGAAAGCTGAGTTGGATCTGGAGGAGCGCGTGATGGGGCCACGCTGTTCACAATGTGACTACATCATGCTACAGAACCTGTCATCTGGGCTGATGCAGAACCTATCAGCTGGGCAGTTGCACCACCAAATATTTGCAACCTATGCAGCTGTGTACAGTGTGGCTCAGGCCCTTCACAACACCCTGCAGTGCAATGTCTCACATTGCCACACATCAGAGCCTGTTCAACCCTGGCAGCTCCTGGAGAACATGTACAATATGAGTTTCCGTGCTCGAGACTTGACACTGCAGTTTGATGCCAAAGGGAGTGTAGACATGGAATATGACCTGAAGATGTGGGTGTGGCAGAGCCCTACACCTGTACTACATACTGTAGGCACCTTCAACGGCACCCTTCAGCTGCAGCACTCGAAAATGTATTGGCCAGGCAACCAGGTGCCAGTCTCCCAGTGCTCCCGGCAGTGCAAAGATGGCCAGGTGCGCAGAGTAAAGGGCTTTCATTCCTGCTGCTATGACTGTGTGGACTGCAAGGCAGGGAGCTACCGGAAGCATCCAGATGACTTCACCTGTACTCCATGTGGCAAGGATCAGTGGTCCCCAGAAAAAAGCACAACCTGCTTACCTCGCAGGCCCAAGTTTCTGGCTTGGGGGGAGCCAGCTGTGCTGTCACTTCTCCTGCTGCTTTGCCTGGTGCTGGGCCTGACACTGGCTGCCCTGGGGCTCTTTGTCCACTACTGGGACAGCCCTCTTGTTCAGGCCTCAGGTGGGTCACTGTTCTGCTTTGGCCTGATCTGCCTAGGCCTCTTCTGCCTCAGTGTCCTTCTGTTCCCAGGACGACCACGCTCTGCCAGCTGCCTTGCCCAACAACCAATGGCTCACCTCCCTCTCACAGGCTGCCTGAGCACACTCTTCCTGCAAGCAGCCGAGATCTTTGTGGAGTCTGAGCTGCCACTGAGTTGGGCAAACTGGCTCTGCAGCTACCTTCGGGGCCCCTGGGCTTGGCTGGTGGTACTGCTGGCCACTCTTGTCGAGGCTGCACTATGTGCCTGGTACTTGATGGCTTTCCCTCCAGAGGTGGTGACAGATTGGCAGGTGCTGCCCACGGAGGTACTGGAACACTGCCGCATGCGTTCCTGGGTCAGCCTGGGCTTGGTGCACATCACCAATGCAGTGTTAGCTTTCCTCTGCTTTCTGGGCACTTTCCTGGTACAGAGCCAGCCTGGTCGCTATAACCGTGCCCGTGGCCTCACCTTCGCCATGCTAGCTTATTTCATCATCTGGGTCTCTTTTGTGCCCCTCCTGGCTAATGTGCAGGTGGCCTACCAGCCAGCTGTGCAGATGGGTGCTATCTTATTCTGTGCCCTGGGCATCCTGGCCACCTTCCACCTGCCCAAATGCTATGTACTTCTGTGGCTGCGAGAGCTCAACACCCAGGAGTTCTTCCTGGGAAGGAGCCCCAAGGAAGCATCAGATGGGAATAGTGGTAGTAGTGAGGCAACTCGGGGACACAGTGAATGA

Also, the following conceptual translations, which correspond to theC-termini of two orthologous pairs of fish T1Rs, are derived fromunpublished genomic sequence fragments and provided. Fugu T1RA wasderived from accession ‘scaffold 164’; Fugu T1RB was derived fromaccession LPC61711; Tetradon T1RA was derived from accession AL226735;Tetradon T1RB was derived from accession AL222381. Ambiguities in theconceptual translations (‘X’) result from ambiguities in databasesequences.

T1RA Fugu SEQ ID NO 12PSPFRDIVSYPDKIILGCFMNLKTSSVSFVLLLLLCLLCFIFSYMGKDLPKNYNEAKAITFCLLLLILTWIIFTTASLLYQGKYIHSLNALAVLSSIYSFLLWYFLPKCYIIIFQPQKNTQKYFQGLIQDYTKTISQ T1RA Tetradon SEQ ID NO 13FAVNYNTPVVRSAGGPMCFLILGCLSLCSISVFFYFERPTEAFCILRFMPFLLFYAVCLACFAVRSFQIVIIFKIAAKFPRVHSWWMKYHGQWLVISMTFVLQAVVIVIGFSSNPPLPYXXFVSYPDKIILGCDVNLNMASTSFFLLLLLCILCFTFSYMGKDLPKNYNEAKAITFCLLLLILTWIIFATAFMLYHGKYIHTLNALAVLSSAYCFLLWYFLPKCYIIIFQPHKNTQKYFQ LST1RB Fugu SEQ ID NO 14KKQGPEVDIFIVSVTILCISVLGVAVGPPEPSQDLDFYMDSIVLECSNTLSPGSFIELCYVCVLSVLCFFFSYMGKDLPANYNEAKCVTFSLMVYMISWISFFTVYLISRGPFTVAAYVCATLVSVLAFFGGYFLPKIYIIVLKPQMNTTAHFQNCIQMYTMSKQ T1RB Tetradon SEQ ID NO 15APKSSQRXLRRTRLXLEWDHPMSVALLFFLVCCLLMTSSSAVILLLNINTPVAKSAGGXTCXLKLAALTAAAMSSXCHFGQPSPLASKLKQPQFTFSFTVCLACNRCALATGHLHFXIRVALPPAYNXWAKNHGPXATIFIASAAILCVLCLRVAVGPPQPSQBLBFXTNSIXLXXSNTLSPGSFVELCNVSLLSAVCFVFSXMGKBLPANYNEAKCVTFSLMVNXISWISFFTVY

Additionally, the accession number and reference citations relating tomouse and rat T1Rs and allelic variants thereof in the public domain areis set forth below:

-   rT1R1 (Accession # AAD18069) (Hoon et al., Cell 96 (4): 541-51    (1999)); rT1R2 (Accession # AAD18070) (Hoon et al., Cell 96(4):    541-59 (1999)); mT1R1 (Accession # AAK39437); mT1R2 (Accession #AAK    39438); mT1R3 (Accession AAK 55537) (Max et al., Nat. Genet. 28(I):    58-63 (2001)); rT1R1 (Accession # AAK7092) (Li et al., Mamm. Genome    (12(I): 13-16 (2001)); mT1R1 (Accession # NP 114073); mT1R1    (Accession # AAK07091) (Li et al., Mamm. Genome (12I):13-16 (2001));    rT1R2 (Accession # AAD18070) (Hoon et al., Cell 9664): 541-551    (1999)); mT1R2 (Accession # NP114079); mT1R3 (Accession # AAK39436);    mT1R3 (Accession # BAB47181); (Kitagawa et al., Biochem. Biophys.    Res. Comm. 283(I):236-42 (2001)); mT1R3 (Accession #NP114078); mT1R3    (Accession # AAK55536) (Max et al., Nat. Genet. 28(I):58-63 (2001));    and mT1R3 (Accession No. AAK01937).

Example 2 Sequence Alignment of Human and Rat T1Rs

Cloned T1R sequences selected from those identified above were alignedagainst the corresponding rat T1Rs. As shown in FIG. 1, human T1R1,human T1R2 and human T1R3 and rat T1R3 were aligned with previouslydescribed T1Rs (rT1R1 having Accession # AAD18069 and rT1R2 havingAccession # AAD18070), the rat mGluR1 metabotropic, glutamate receptor(Accession # P23385); and the human calcium-sensing receptor (Accession#P41180). For clarity of the comparison, the mGluR1 and calcium-sensingreceptor C-termini are truncated. The seven potential transmembranesegments are boxed in blue. Residues that contact the glutamateside-chain carbutylate in the mGluR1 crystal structure are boxed in red,and residues that contact the glutamate α-amino acid moiety are boxed ingreen. The mGluR1 and calcium-sensing receptor cysteine residuesimplicated in intersubunit disulfide-based formation are circled inpurple. These cysteines are not conserved in T1R1 and T1R2, but arelocated in a degraded region of the alignment that contains apotentially analogous T1R3 cysteine residue, also circled.

Example 3 Demonstration by RT-PCR that hT1R2 and hT1R3 are Expressed inTaste Tissue

As shown in FIG. 2, hT1R2 and hT1R3 are expressed in taste tissue:expression of both genes can be detected by RT-PCR from resected humancircumvallate papillae.

Example 4 Methods for Heterologous Expression of T1Rs in HeterologousCells

An HEK-293 derivative (Chandrashekar et al., Cell 100(6): 703-11(2000)), which stably expresses Gα15, was grown and maintained at 37° C.in Dulbecco's Modified Eagle Medium (DMEM, Gibco BRL) supplemented with10% FBS, MEM non-essential amino acids (Gibco BRL), and 3 μg/mlblasticidin. For calcium-imaging experiments, cells were first seededonto 24-well tissue-culture plates (approximately 0.1 million cells perwell), and transfected by lipofection with Mirus Transit-293 (PanVera).To minimize glutamate-induced and glucose-induced desensitization,supplemented DMEM was replaced with low-glucose DMEM/GlutaMAX (GibcoBRL) approximately 24 hours after transfection. 24 hours later, cellswere loaded with the calcium dye Fluo-4 (Molecular Probes), 3 μM inDulbecco's PBS buffer (DPBS, GibcoBRL), for 1.5 hours at roomtemperature. After replacement with 250 μl DPBS, stimulation wasperformed at room temperature by addition of 200 μl DPBS supplementedwith taste stimuli. Calcium mobilization was monitored on a AxiovertS100 TV microscope (Zeiss) using Imaging Workbench 4.0 software (Axon).T1R1/T1R3 and T1R2/T1R3 responses were strikingly transient—calciumincreases rarely persisted longer than 15 seconds—and asynchronous. Thenumber of responding cells was thus relatively constant over time;therefore, cell responses were quantitated by manually counting thenumber of responding cells at a fixed time point, typically 30 secondsafter stimulus addition.

Example 5 Human T1R2/T1R3 Functions as a Sweet Taste Receptor

HEK cells stably expressing Gα15 were transiently transfected with humanT1R2, T1R3 and T1R2/T1R3, and assayed for increases in intracellularcalcium in response to increasing concentrations of sucrose (FIG. 3(a)). Also, T1R2/T1R3 dose responses were determined for several sweettaste stimuli (FIG. 3( b)). The maximal percentage of responding cellswas different for different sweeteners, ranging from 10-30%. Forclarity, dose responses were normalized to the maximal percentage ofresponding cells. The values in FIG. 3 represent the mean±s.e. of fourindependent responses. X-axis circles mark psychophysical detectionthresholds determined by taste testing. Gurmarin (50-fold dilution of afiltered 10 g/l Gymnema sylvestre aqueous extract) inhibited theresponse of T1R2/T1R3 to 250 mM sucrose, but not the response ofendogenous β2-adrenergic receptor to 20 μM isoproterenol (FIG. 3( b)).FIG. 3( c) contains the normalized response of T1R2/T1R3 co-expressingcell lines to different sweeteners (sucrose, aspartame, D-tryptophan andsaccharin).

Example 6 Rat T1R2/T1R3 Also Functions as a Sweet Taste Receptor

HEK cells stably expressing Gα15 were transiently transfected withhT1R2/hT1R3, rT1R2/rT1R3, hT1R2/rT1R3, and rT1R2/hT1R3. Thesetransfected cells were then assayed for increased intracellular calciumin response to 350 mM sucrose, 25 mM tryptophan, 15 mM aspartame, and0.05% of monellin. The results with sucrose and aspartame are containedin FIG. 4 and indicate that rT1R2/rT1R3 also functions as a sweet tastereceptor. Also, these results suggest that T1R2 may control T1R2/T1R3ligand specificity.

Example 7 T1R2/T1R3 Responses Using an Automated Fluorescence BasedAssay

HEK cells stably expressing Gα15 were transiently transfected with hT1R2and hT1R3. These cells were loaded with the calcium dye Fluo-4, andtheir responses to a sweetener measured using a fluorescence platereader. FIG. 5 contains cyclamate (12.5 mM) responses for cellsexpressing hT1R2/hT1R3 and for cells expressing only hT1R3 (J19-22). Thefluorescence results obtained indicate that responses to these tastestimuli only occurred in the cells expressing hT1R2/hT1R3. FIG. 6contains normalized dose-response curves, the results of which show thathT1R2 and hT1R3 function together as a human taste receptor based ontheir dose-specific interaction with various sweet stimuli.Particularly, FIG. 6 contains dose-responses for sucrose, tryptophan andvarious other commercially available sweeteners. These results indicatethat T1R2/T1R3 is a human sweet taste receptor as the rank order andthreshold values obtained in the assay closely mirror values for humansweet taste.

Example 8 Ligand-binding Residues of mGluR1 are Conserved in T1R1

As shown in FIG. 6, the key ligand-binding residues of mGluR1 areconserved in T1R1. The interaction of glutamate with mGluR1 is shownwith several key residues highlighted according to the same color schemeas FIG. 1.

Example 9 Human T1R1/T1R3 Functions as Umami Taste Receptors

HEK cells stably expressing Gα15 were transiently transfected with humanT1R1, T1R3 and T1R1/T1R3 and assayed for increases in intracellularcalcium in response to increasing concentrations of glutamate (FIG. 8(a)), and 0.5 mM glutamate), 0.2 mM IMP, and 0.5 mM glutamate plus 0.2 mMIMP (FIG. 8( b)). Human T1R1/T1R3 dose responses were determined forglutamate in the presence and absence of 0.2 mM IMP (FIG. 8( c)). Themaximal percentages of responding cells was approximately 5% forglutamate and approximately 10% for glutamate plus IMP. For clarity,does responses are normalized to the maximal percentage of respondingcells. The values represent the mean±s.e. of four independent responses.X-axis circles mark taste detection thresholds determined by tastetesting.

Example 10 PDZIP as an Export Sequence

The six residue PDZIP sequence (SVSTW (SEQ ID NO:1)) was fused to theC-terminus of hT1R2 and the chimeric receptor (i.e. hT1R2-PDZIP) wastransfected into an HEK-293 host cell. The surface expression of hT1R2was then monitored using immunofluorescence and FACS scanning data. Asshown in FIGS. 9A and 9B, the inclusion of the PDZIP sequence increasedthe surface expression of hT1R2-PDZ1P relative to hT1R2. Morespecifically, FIG. 9A shows an immunofluorescence staining of myc-taggedhT1R2 demonstrating that PDZIP significantly increases the amount ofhT1R2 protein on the plasma membrane. FIG. 9B shows FACS analysis datademonstrating the same result—Cells expressing myc-tagged hT1R2 areindicated by the dotted line and cells expressing myc-tagged hT1R2-PDZIPare indicated by the solid line. Particularly, FIG. 10A showsuntransfected Gα15 stable host cells in HBS buffer, FIG. 10B showshT1R2-PDZIP transfected Gα15 stable hose cells in sweetener pool no. 5(saccharin, sodium cyclamate, Acesulfame K, and Aspartame-20 mM each inHBS buffer), FIG. 10C shows T1R3-PDZIP transfected Gα15 stable hostcells in sweetener pool no. 5, and FIG. 10D showshT1R2-PDZIP/hT1R3-PDZIP co-transfected Gα15 stable host cells insweetener pool no. 5. Further, FIGS. 10E-10H show dose-dependentresponse of hT1R2/hT1R3 co-transfected Gα15 stable host cells tosucrose—E: 0 mM in HBS buffer; F: 30 mM; G: 60 mM; and H: 250 mM. FIGS.10I-10L shown the responses of hT1R2/hT1R3 co-transfected Gα15 stablehost cells to individual sweeteners—I: Aspartame (1.5 mM); J: AcesulfameK (1 mM); K: Neotame (20 mM); L: Sodium cyclamate (20 mM). Asdemonstrated by the calcium-images of FIG. 10, hT1R2 and hT1R3 are bothrequired for the activities triggered by the sweet stimuli.

Example 11 Generation of Cell Lines that Stably Co-Express T1R1/T1R3 orT1R2/T1R3

Human cell lines that stably co-express human T1R2/T1R3 or humanT1R1/T1R3 were generated by transfecting linearized PEAK10-derived (EdgeBiosystems) vectors and pCDNA 3.1/ZEO-derived (Invitrogen) vectorsrespectively containing hT1R1 or hT1R2 expression construct (plasmidSAV2485 for T1R1, SAV2486 for T1R2) and hT1R3 (plasmid SXV550 for T1R3)into a G_(α15) expressing cell line. Specifically, T1R2/T1R3 stable celllines were produced by co-transfecting linearized SAV2486 and SXV550into Aurora Bioscience's HEK-293 cell line that stably expressesG_(α15). T1R1/T1R3 stable cell lines were produced by co-transfectinglinearized SAV2485 and SXV550 into the same HEK-293 cell line thatstably expresses G_(α15). Following SAV2485/SXV550 and SAV2486/SXV550transfections, puromycin-resistant and zeocin-resistant colonies wereselected, expanded, and tested by calcium imaging for responses to sweetor umami taste stimuli. Cells were selected in 0.0005 mg/ml puromycin(CALBIOCHEM) and 0.1 mg/ml zeocin (Invitrogen) at 37° C. in low-glucoseDMEM supplemented with GlutaMAX, 10% dialyzed FBS, and 0.003 mg/mlblasticidin. Resistant colonies were expanded, and their responses tosweet taste stimuli evaluated by Fluorescence microscopy. For automatedfluorimetric imaging on VIPR-II instrumentation (Aurora Biosciences),T1R2/T1R3 stable cells were first seeded onto 96-well plates(approximately 100,000 cells per well). Twenty-four hours later, cellswere loaded with the calcium dye fluo-3-AM (Molecular Probes), 0.005 mMin PBS, for one hour at room temperature. After replacement with 70 μlPBS, stimulation was performed at room temperature by addition of 70 μlPBS supplemented with taste stimuli. Fluorescence (480 nm excitation and535 nm emission) responses from 20 to 30 seconds following compoundaddition were averaged, corrected for background fluorescence measuredprior to compound addition, and normalized to the response to 0.001 mMionomycin (CALBIOCHEM), a calcium ionophore.

It was then observed that when these cell lines were exposed to sweet orumami stimuli, that for active clones typically 80-100% of cellsresponded to taste stimuli. Unexpectedly, the magnitude of individualcell responses was markedly larger than that of transiently transfectedcells.

Based on this observation, the inventors tested the activity of T1Rstable cell lines by automated fluorescence imaging using AuroraBioscience's VIPR instrumentation as described above. The responses oftwo T1R1/T1R3 and one T1R2/T1R3 cell line are shown in FIG. 11 and FIG.12 respectively.

Remarkably, the combination of increased numbers of responding cells andincreased response magnitudes resulted in a greater than 10-foldincrease in activity relative to transiently transfected cells. (By wayof comparison, the percent ionomycin response for cells transientlytransfected with T1R2/T1R3 was approximately 5% under optimalconditions.) Moreover, dose responses obtained for stably expressedhuman T1R2/T1R3 and T1R1/T1R3 correlated with human taste detectionthresholds. The robust T1R activity of these stable cell lines suggeststhat they are well suited for use in high-throughput screening ofchemical libraries in order to identify compounds, e.g. small molecules,that modulate the sweet or umami taste receptor and which thereforemodulate, enhance, block or mimic sweet or umami taste.

Example 12 Generation of Cell Lines that Inducibly Co-express T1R1/T1R3which Selectively Respond to Umami Taste Stimuli

T1R1/T1R3 HEK 293 cell lines that stably expressed the umami tastereceptor display robust improved activity relative to transientlytranfected cites. However, a disadvantage is that they can rapidly loseactivity during cell propagation.

Also, these findings support the inventors' hypothesis that (i)T1R1/T1R3 is a umami taste receptor, i.e., and (ii) that cell lineswhich robustly express T1R1/T1R3, preferably stable and/or inducibleT1R1/T1R3 cell lines can be used in assays, preferably for highthroughput screening of chemical libraries to identify novel modulatorsof umami taste. Modulators that enhance umami taste may be used.

To overcome the instability of the T1R1/T1R3 stable cell lines, theHEK-G_(α15) cells have been engineered to inducibly express T1R1/T1R3using the GeneSwitch system (Invitrogen). pGene-derived zeocin-resistantexpression vectors for human T1R1 and T1R3 (plasmid SXV603 for T1R1 andSXV611 for T1R3) and a puromycin-resistant pSwitch-derived vector thatcarries the GeneSwitch protein (plasmid SXV628) were linearized andcotransfected into the HEK-G_(α15) cell line. Zeocin-resistant andpuromycin-resistant colonies were selected, expanded, induced withvariable amounts of mifepristone, and tested by calcium imaging forresponses to umami taste stimuli.

Inducible expression of T1R1/T1R3 resulted in robust activity. Forexample, approximately 80% of induced cells but only approximately 10%of transiently transfected cells responded to L-glutamate; Morespecifically, pGene derived Zeocin-resistant expression vectors thatexpress human T1R1 and human T1R3 and a puromycn-resistantpSwitch-derived vector that carries the GeneSwitch protein werelinearized and co-tranfected into G_(α15) cells. Cells were selected in0.5 μg/ml puromycin (CAL BIOCHEM) and 100 μg/ml Zeocin (Invitrogen) at37° C. in Dulbecco's Modified Eagle Medium supplemented with GlutaMAX,(10% dialyzed FBS, and 3 ug/ml blasticidin. Resistant colonies wereexpanded, and their responses to umami taste stimuli following inductionwith 10⁻¹⁰ M mifepristone determined by fluorescence microscopyfollowing the methods of Li et al., PNAS 99(7): 4692-4696 (2002).

For automated fluorometric imaging on FLIPR instrumentation (MolecularDevice), cells from one clone (designated clone I-17) were seeded into96-well plates (approximately 80,000 cell per well) in the presence of10⁻¹⁰ M mifepristone and incubated for 48 hours. Cells were then loadedwith the calcium dye fluo-4-AM (Molecular Probes), 3 μM in PBS, for 1.5hours at room temperature.

After replacement with 50 μl PBS, stimulation was performed at roomtemperature by the addition of 50 μl PBS supplemented with differentstimuli. In contrast to previous transient T1R1/T1R3 umami receptorexpression systems that necessitated quantifying T1R1/T1R3 receptoractivity by individually counting responding cells (Li et al., PNAS99(7): 4692-4696 (2002)) (because of the low activity of the receptortherein), the subject inducible expression system resulted in a cloneI-17 having substantially increased activity that allowed receptoractivity to be quantified by determining maximal fluorescence increases(480 nm excitation and 535 nm emission) summated over fields of imagedcells. The maximal fluorescence from four independent determinationswere averaged, corrected for background fluorescence measured prior tocompound addition, and normalized to the response to 0.002 mM ionomycin(CALBIOCHEM).

These results are contained in FIG. 13. Particularly, FIG. 13 contains adose-response curve determined for L-glutamate in the presence andabsence of 0.2 mM IMP. In the figure, each value represents averagesummated maximal fluorescence (corrected for background fluorescence)for four independent determinations. These dose-response curvescorrespond to those determined for cells transiently transfected withT1R1/T1R3.

The selectivity of the umami T1R1/T1R3 taste receptor was also evaluatedby screening with different L-amino acids. The results obtainedindicated that T1R1/T1R3 is selectively activated by the umami-tastingL-amino acids (L-glutamate and L-aspartate).

The results of experiments wherein the responses of the I-17 clone wasresulted in tested in the presence of different L-amino acids arecontained in FIG. 14 and FIG. 15. FIG. 14 shows the results of anexperiment wherein the I-17 cell line was contacted with differentL-amino acids at a concentration of 10 mM in the presence and absence of1 mM IMP.

FIG. 15 contains a dose-response curve for active amino acids determinedin the presence of 0.2 mM IMP. Each value represents the average of fourindependent determinations.

The results obtained in these experiments support the specificity andselectivity of the umami taste receptor to umami taste stimuli. Whereasthe umami taste stimuli L-glutamate and L-aspartate significantlyactivated the T1R1/T1R3 receptor at different concentrations (see FIGS.14 and 15), the other L-amino acids which activated the human T1R1/T1R3receptor only activated the receptor weakly and at much higherconcentrations.

Therefore, these results support the selectivity of the T1R1/T1R3receptor for umami taste stimuli and the suitability of this induciblestable expression system for use in high throughput screening assaysusing automated fluorometric imaging instrumentation to identifycompounds that activate the umami taste receptor, for exampleL-glutamate or L-aspartate, or which enhance the activity of L-glutamateto activate the umami taste receptor, for example 5′-IMP or 5′-GMP, orblock the activation of the umami taste receptor by umami taste stimulisuch as L-glutamate and L-aspartate.

Compounds identified using these assays have potential application asflavorants in foods and beverage compositions for mimicing or blockingumami taste stimuli.

Example 13 Lactisole Inhibits the Receptor Activities of Human T1R2/T1R3and T1R1/T1R3, and Sweet and Umami Taste

Lactisole, an aralkyl carboxylic acid, was thought to be a selectivesweet-taste inhibitor (See e.g., Lindley (1986) U.S. Pat. No. 4,567,053;and Schiffman et al. Chem Senses 24:439-447 (1999)). Responses ofHEK-G_(α15) cells transiently transfected with T1R2/T1R3 to 150 mMsucrose in the presence of variable concentrations of lactisole weremeasured. Lactisole inhibits the activity of human T1R2/T1R3 with anIC₅₀ of 24 μM.

The T1R1/T1R3 umami and T1R2/T1R3 sweet taste receptor may share acommon subunit. It has therefore been theorized that lactisole, whichinhibit the T1R2/T1R3 sweet taste receptor, may have a similar effect onthe T1R1/T1R3 umami taste receptor. The present inventors tested theeffect of lactisole on the response of human T1R1 /T1R3 to 10 mML-Glutamate. As with the T1R2/T1R3 sweet receptor, lactisole inhibitedT1R1/T1R3 with an IC₅₀ of 165 μM. Lactisole inhibition likely reflectsantagonism at the T1R receptors instead of, for example, non-specificinhibition of G_(α15)-mediated signaling because the response ofmuscarinic acetylcholine receptors was not inhibited by lactisole.

The present inventors then evaluated the effect of lactisole on humanumami taste. Taste thresholds in the presence of 1 and 2 mM lactisolewere determined for the umami taste stimuli L-Glutamate with or without0.2 mM IMP, the sweet taste stimuli sucrose and D-tryptophan, and thesalty taste stimulus sodium chloride following the methods of Schiffmanet al. (Chem. Senses 24: 439-447 (1989)). Millimolar concentrations oflactisole dramatically increased detection thresholds for sweet andumami but not salt taste stimuli. These results are contained in FIG.16.

In conclusion, (i) these findings further support the inventors'hypothesis that T1R1/T1R3 is the only umami taste receptor, and (ii) theT1R1/T1R3 and T1R2/T1R3 receptors may share a structurally relatedlactisole-binding domain.

While the foregoing detailed description has described severalembodiments of the present invention, it is to be understood that theabove description is illustrative only and not limiting of the disclosedinvention. The invention is to be limited only by the claims whichfollow.

1. An in vitro method of detecting a cell that is potentially sensitiveto sweet tastants, the method comprising: (a) detecting the expressionof a T1R2 polypeptide and/or a nucleic acid encoding said T1R2polypeptide by said cell by contacting said cell with at least one probethat detects the expression of said T1R2 polypeptide and/or said nucleicacid encoding said T1R2 polypeptide by said cell, wherein said T1R2polypeptide (i) is encoded by a nucleic acid sequence comprising SEQ IDNO: 10, or (ii) possesses at least 90% sequence identity to the T1R2polypeptide of SEQ ID NO: 6; (b) detecting the expression of a T1R3polypeptide and/or a nucleic acid encoding said T1R3 polypeptide by saidcell by contacting said cell with at least one probe that detects theexpression of said T1R3 polypeptide and/or said nucleic acid encodingsaid T1R3 polypeptide by said cell, wherein said T1R3 polypeptide (i) isencoded by a nucleic acid sequence comprising SEQ ID NO: 9; or (ii)possesses at least 90% sequence identity to the T1R3 polypeptide of SEQID NO: 7; and (c) based on the results of (a) and (b), identifying acell that is detected to express T1R2 and T1R3 as a cell that ispotentially sensitive to sweet tastants (“detected cell”).
 2. The methodof claim 1, further comprising confirming that the detected cell issensitive to sweet tastants, and using said detected cell in ahigh-throughput screening assay that detects compounds that specificallybind, activate or modulate the activation of a receptor comprising saidT1R2 and T1R3 polypeptides, or which modulate the binding or activationof a receptor comprising said T1R2 and T1R3 polypeptides by anothercompound.
 3. The method of claim 2, wherein the detected cell isisolated.
 4. The method of claim 1, wherein said detected cell isisolated from a mammalian tissue.
 5. The method of claim 2, wherein saiddetected cell is isolated from a mammalian tissue.
 6. The method ofclaim 1, wherein the detected cell is isolated from a monkey tissue. 7.The method of claim 2, wherein the detected cell is isolated from amonkey tissue.
 8. The method of claim 1, wherein the detected cell isisolated from a human tissue.
 9. The method of claim 2, wherein thedetected cell is isolated from a human tissue.
 10. The method of claim4, wherein said mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 11. The method of claim 5,wherein said mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 12. The method of claim 6,wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 13. The method of claim 7,wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 14. The method of claim 8,wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 15. The method of claim 9,wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 16. The method of claim 1,wherein the detected cell is isolated from a mammalian cell culture. 17.The method of claim 2, wherein the detected cell is isolated from amammalian cell culture.
 18. The method of claim 1, wherein the detectedcell is isolated from a human cell culture.
 19. The method of claim 2,wherein the detected cell is isolated from a human cell culture.
 20. Themethod of claim 1, wherein the detected cell is isolated from a monkeycell culture.
 21. The method of claim 2, wherein the detected cell isisolated from a monkey cell culture.
 22. The method of claim 1, whereinstep (b) comprises detecting the expression of T1R3 by said cell bycontacting said cell with at least one probe that detects the expressionof said nucleic acid encoding said T1R3 polypeptide by said cell. 23.The method of claim 1, wherein step (a) comprises detecting theexpression of T1R2 by said cell by contacting said cell with at leastone probe that detects the expression of said nucleic acid encoding saidT1R2 polypeptide by said cell.
 24. The method of claim 1, wherein thedetected cell is transfected with a heterologous nucleic acid encoding adetectable label that provides for the identification of compounds thatspecifically bind or modulate the activity of a receptor comprising saidT1R2 polypeptide and said T1R3 polypeptide.
 25. The method of claim 1,wherein the detected cell endogenously expresses a G protein or istransfected with a heterologous nucleic acid encoding a G protein. 26.The method of claim 25, wherein said G protein comprises gustducin, apromiscuous G protein, Gα15, Gα16, or a chimeric G protein.
 27. Themethod of claim 1, wherein said T1R2 polypeptide and/or said nucleicacid encoding said T1R2 polypeptide is expressed from a gene that isendogenous to said detected cell.
 28. The method of claim 1, whereinsaid T1R3 polypeptide and/or said nucleic acid encoding said T1R3polypeptide is expressed from a gene that is endogenous to said detectedcell.
 29. The method of claim 1, wherein said T1R2 polypeptide and/orsaid nucleic acid encoding said T1R2 polypeptide is expressed from agene that is endogenous to said cells and said T1R3 polypeptide and/orsaid nucleic acid encoding said T1R3 polypeptide is expressed from agene that is endogenous to said detected cell.
 30. The method of claim27, wherein said detected cell is a mammalian, human, or monkey cell.31. The method of claim 28, wherein said detected cell is a mammalian,human, or monkey cell.
 32. The method of claim 29, wherein said detectedcell is a mammalian, human, or monkey cell.
 33. The method of claim 1,wherein said at least one probe in step (a) comprises a nucleic acidthat specifically detects said nucleic acid encoding said T1R2polypeptide or comprises an antibody that specifically detects said T1R2polypeptide and/or wherein said at least one probe in step (b) comprisesa nucleic acid that specifically detects said nucleic acid encoding saidT1R3 polypeptide or comprises an antibody that specifically detects saidT1R3 polypeptide.
 34. The method of claim 33, wherein said nucleic acidencoding said T1R2 polypeptide and/or said nucleic acid encoding saidT1R3 polypeptide is an RNA and is detected by a method selected from thegroup consisting of Northern analysis, dot blot, RNase protection, S1analysis, a nucleic acid amplification technique and in situhybridization.
 35. The method of claim 34, wherein said at least oneprobe in step (a) comprises at least one probe that detects theexpression of said nucleic acid encoding said T1R2 polypeptide by saidcell and wherein said at least one probe in step (b) comprises at leastone probe that detects the expression of said nucleic acid encoding saidT1R3 polypeptide by said cell.
 36. The method of claim 34, wherein saidat least one probe in step (a) comprises at least one probe that detectsthe expression of said nucleic acid encoding said T1R2 polypeptide bysaid cell.
 37. The method of claim 34, wherein said at least one probein step (b) comprises at least one probe that detects the expression ofsaid nucleic acid encoding said T1R3 polypeptide by said cell.
 38. Themethod of claim 34, wherein the nucleic acid amplification techniquecomprises a polymerase chain reaction amplification.
 39. The method ofclaim 1, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallyactivate or modulate sweet taste signaling.
 40. The method of claim 39,wherein said high-throughput screening assay detects activators,inhibitors, blockers, stimulators, enhancers, agonists, or antagonistsof sweet taste signaling.
 41. The method of claim 1, further comprisingusing said detected cell in a high-throughput screening assay thatdetects compounds that specifically bind, activate or modulate theactivation of a receptor comprising said T1R2 and T1R3 polypeptides, orwhich modulate the binding or activation of a receptor comprising saidT1R2 and T1R3 polypeptides by another compound.
 42. The method of claim1, wherein said T1R2 polypeptide and/or said nucleic acid encoding saidT1R2 polypeptide is expressed from a nucleic acid heterologous to saiddetected cell.
 43. The method of claim 1, wherein said T1R3 polypeptideand/or said nucleic acid encoding said T1R3 polypeptide is expressedfrom a nucleic acid heterologous to said detected cell.
 44. An in vitromethod of detecting a cell that is sensitive to sweet tastants, themethod comprising: (a) detecting the expression of a T1R2 polypeptideand/or a nucleic acid encoding said T1R2 polypeptide by said cell bycontacting said cell with at least one probe that detects the expressionof said T1R2 polypeptide and/or said nucleic acid encoding said T1R2polypeptide by said cell, wherein said T1R2 polypeptide (i) is encodedby a nucleic acid sequence comprising SEQ ID NO: 10, or (ii) possessesat least 90% sequence identity to the T1R2 polypeptide of SEQ ID NO: 6;(b) detecting the expression of a T1R3 polypeptide and/or a nucleic acidencoding said T1R3 polypeptide by said cell by contacting said cell withat least one probe that detects the expression of said T1R3 polypeptideand/or said nucleic acid encoding said T1R3 polypeptide by said cell,wherein said T1R3 polypeptide (i) is encoded by a nucleic acid sequencecomprising SEQ ID NO: 9; or (ii) possesses at least 90% sequenceidentity to the T1R3 polypeptide of SEQ ID NO: 7; (c) based on theresults of (a) and (b), identifying a cell that is detected to expressT1R2 and T1R3 as a cell that is potentially sensitive to sweet tastants(“detected cell”); and (d) confirming that the detected cell issensitive to sweet tastants.
 45. The method of claim 44, wherein thedetected cell is isolated from a mammalian tissue.
 46. The method ofclaim 44, wherein the detected cell is isolated from a monkey tissue.47. The method of claim 44, wherein the detected cell is isolated from ahuman tissue.
 48. The method of claim 45, wherein said mammalian tissuecomprises tongue, oral cavity tissue, gastrointestinal epithelium, orepiglottis.
 49. The method of claim 46, wherein said monkey tissuecomprises tongue, oral cavity tissue, gastrointestinal epithelium, orepiglottis.
 50. The method of claim 47, wherein said human tissuecomprises tongue, oral cavity tissue, gastrointestinal epithelium, orepiglottis.
 51. The method of claim 44, wherein the detected cell isisolated from a mammalian cell culture.
 52. The method of claim 44,wherein the detected cell is isolated from a human cell culture.
 53. Themethod of claim 44, wherein the detected cell is isolated from a monkeycell culture.
 54. The method of claim 44, wherein step (b) comprisesdetecting the expression of T1R3 by said cell by contacting said cellwith at least one probe that detects the expression of said nucleic acidencoding said T1R3 polypeptide by said cell.
 55. The method of claim 44,wherein step (a) comprises detecting the expression of T1R2 by said cellby contacting said cell with at least one probe that detects theexpression of said nucleic acid encoding said T1R2 polypeptide by saidcell.
 56. The method of claim 44, wherein the detected cell istransfected with a heterologous nucleic acid encoding a detectable labelthat provides for the identification of compounds that specifically bindor modulate the activity of a receptor comprising said T1R2 polypeptideand said T1R3 polypeptide.
 57. The method of claim 44, wherein thedetected cell endogenously expresses a G protein or is transfected witha heterologous nucleic acid encoding a G protein.
 58. The method ofclaim 57, wherein said G protein comprises gustducin, a promiscuous Gprotein, Gα15, Gα16, or a chimeric G protein.
 59. The method of claim44, wherein said T1R2 polypeptide and/or said nucleic acid encoding saidT1R2 polypeptide is expressed from a gene that is endogenous to saiddetected cell.
 60. The method of claim 44, wherein said T1R3 polypeptideand/or said nucleic acid encoding said T1R3 polypeptide is expressedfrom a gene that is endogenous to said detected cell.
 61. The method ofclaim 44, wherein said T1R2 polypeptide and/or said nucleic acidencoding said T1R2 polypeptide is expressed from a gene that isendogenous to said cells and said T1R3 polypeptide and/or said nucleicacid encoding said T1R3 polypeptide is expressed from a gene that isendogenous to said detected cell.
 62. The method of claim 59, whereinsaid detected cell is a mammalian, human, or monkey cell.
 63. The methodof claim 60, wherein said detected cell is a mammalian, human, or monkeycell.
 64. The method of claim 61, wherein said detected cell is amammalian, human, or monkey cell.
 65. The method of claim 44, whereinsaid at least one probe in step (a) comprises a nucleic acid thatspecifically detects said nucleic acid encoding said T1R2 polypeptide orcomprises an antibody that specifically detects said T1R2 polypeptideand/or wherein said at least one probe in step (b) comprises a nucleicacid that specifically detects said nucleic acid encoding said T1R3polypeptide or comprises an antibody that specifically detects said T1R3polypeptide.
 66. The method of claim 65, wherein said nucleic acidencoding said T1R2 polypeptide and/or said nucleic acid encoding saidT1R3 polypeptide is an RNA and is detected by a method selected from thegroup consisting of Northern analysis, dot blot, RNase protection, S1analysis, a nucleic acid amplification technique and in situhybridization.
 67. The method of claim 66, wherein said at least oneprobe in step (a) comprises at least one probe that detects theexpression of said nucleic acid encoding said T1R2 polypeptide by saidcell and wherein said at least one probe in step (b) comprises at leastone probe that detects the expression of said nucleic acid encoding saidT1R3 polypeptide by said cell.
 68. The method of claim 66, wherein saidat least one probe in step (a) comprises at least one probe that detectsthe expression of said nucleic acid encoding said T1R2 polypeptide bysaid cell.
 69. The method of claim 66, wherein said at least one probein step (b) comprises at least one probe that detects the expression ofsaid nucleic acid encoding said T1R3 polypeptide by said cell.
 70. Themethod of claim 66, wherein the nucleic acid amplification techniquecomprises a polymerase chain reaction amplification.
 71. The method ofclaim 44, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallyactivate or modulate sweet taste signaling.
 72. The method of claim 71,wherein said high-throughput screening assay detects activators,inhibitors, blockers, stimulators, enhancers, agonists, or antagonistsof sweet taste signaling.
 73. The method of claim 44, further comprisingusing said detected cell in a high-throughput screening assay thatdetects compounds that specifically bind, activate or modulate theactivation of a receptor comprising said T1R2 and T1R3 polypeptides, orwhich modulate the binding or activation of a receptor comprising saidT1R2 and T1R3 polypeptides by another compound.
 74. The method of claim44, wherein said T1R2 polypeptide and/or said nucleic acid encoding saidT1R2 polypeptide is expressed from a nucleic acid heterologous to saiddetected cell.
 75. The method of claim 44, wherein said T1R3 polypeptideand/or said nucleic acid encoding said T1R3 polypeptide is expressedfrom a nucleic acid heterologous to said detected cell.
 76. An in vitromethod of detecting an isolated cell that is potentially sensitive tosweet tastants, the method comprising: (a) detecting the expression of aT1R2 polypeptide and/or a nucleic acid encoding said T1R2 polypeptide bysaid isolated cell by contacting said isolated cell with at least oneprobe that detects the expression of said T1R2 polypeptide and/or saidnucleic acid encoding said T1R2 polypeptide by said isolated cell,wherein said T1R2 polypeptide (i) is encoded by a nucleic acid sequencecomprising SEQ ID NO: 10, or (ii) possesses at least 90% sequenceidentity to the T1R2 polypeptide of SEQ ID NO: 6; (b) detecting theexpression of a T1R3 polypeptide and/or a nucleic acid encoding saidT1R3 polypeptide by said isolated cell by contacting said isolated cellwith at least one probe that detects the expression of said T1R3polypeptide and/or said nucleic acid encoding said T1R3 polypeptide bysaid isolated cell, wherein said T1R3 polypeptide (i) is encoded by anucleic acid sequence comprising SEQ ID NO: 9; or (ii) possesses atleast 90% sequence identity to the T1R3 polypeptide of SEQ ID NO: 7; and(c) based on the results of (a) and (b), identifying a cell that isdetected to express T1R2 and T1R3 as a cell that is potentiallysensitive to sweet tastants (“detected cell”).
 77. The method of claim76, further comprising confirming that the detected cell is sensitive tosweet tastants.
 78. The method of claim 76, wherein the detected cell isisolated from a mammalian tissue.
 79. The method of claim 77, whereinthe detected cell is isolated from a mammalian tissue.
 80. The method ofclaim 76, wherein the detected cell is isolated from a monkey tissue.81. The method of claim 77, wherein the detected cell is isolated from amonkey tissue.
 82. The method of claim 76, wherein the detected cell isisolated from a human tissue.
 83. The method of claim 77, wherein thedetected cell is isolated from a human tissue.
 84. The method of claim78, wherein said mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 85. The method of claim 79,wherein said mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 86. The method of claim 80,wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 87. The method of claim 81,wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 88. The method of claim 82,wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 89. The method of claim 83,wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 90. The method of claim 76,wherein the detected cell is isolated from a mammalian cell culture. 91.The method of claim 77, wherein the detected cell is isolated from amammalian cell culture.
 92. The method of claim 76, wherein the detectedcell is isolated from a human cell culture.
 93. The method of claim 77,wherein the detected cell is isolated from a human cell culture.
 94. Themethod of claim 76, wherein the detected cell is isolated from a monkeycell culture.
 95. The method of claim 77, wherein the detected cell isisolated from a monkey cell culture.
 96. The method of claim 76, whereinstep (b) comprises detecting the expression of T1R3 by said cell bycontacting said cell with at least one probe that detects the expressionof said nucleic acid encoding said T1R3 polypeptide by said cell. 97.The method of claim 76, wherein step (a) comprises detecting theexpression of T1R2 by said cell by contacting said cell with at leastone probe that detects the expression of said nucleic acid encoding saidT1R2 polypeptide by said cell.
 98. The method of claim 76, wherein thedetected cell is transfected with a heterologous nucleic acid encoding adetectable label that provides for the identification of compounds thatspecifically bind or modulate the activity of a receptor comprising saidT1R2 polypeptide and said T1R3 polypeptide.
 99. The method of claim 76,wherein the detected cell endogenously expresses a G protein or istransfected with a heterologous nucleic acid encoding a G protein. 100.The method of claim 99, wherein said G protein comprises gustducin, apromiscuous G protein, Gα15, Gα16, or a chimeric G protein.
 101. Themethod of claim 76, wherein said T1R2 polypeptide and/or said nucleicacid encoding said T1R2 polypeptide is expressed from a gene that isendogenous to said detected cell.
 102. The method of claim 76, whereinsaid T1R3 polypeptide and/or said nucleic acid encoding said T1R3polypeptide is expressed from a gene that is endogenous to said detectedcell.
 103. The method of claim 76, wherein said T1R2 polypeptide and/orsaid nucleic acid encoding said T1R2 polypeptide is expressed from agene that is endogenous to said cells and said T1R3 polypeptide and/orsaid nucleic acid encoding said T1R3 polypeptide is expressed from agene that is endogenous to said detected cell.
 104. The method of claim101, wherein said detected cell is a mammalian, human, or monkey cell.105. The method of claim 102, wherein said detected cell is a mammalian,human, or monkey cell.
 106. The method of claim 103, wherein saiddetected cell is a mammalian, human, or monkey cell.
 107. The method ofclaim 76, wherein said at least one probe in step (a) comprises anucleic acid that specifically detects said nucleic acid encoding saidT1R2 polypeptide or comprises an antibody that specifically detects saidT1R2 polypeptide and/or wherein said at least one probe in step (b)comprises a nucleic acid that specifically detects said nucleic acidencoding said T1R3 polypeptide or comprises an antibody thatspecifically detects said T1R3 polypeptide.
 108. The method of claim107, wherein said nucleic acid encoding said T1R2 polypeptide and/orsaid nucleic acid encoding said T1R3 polypeptide is an RNA and isdetected by a method selected from the group consisting of Northernanalysis, dot blot, RNase protection, S1 analysis, a nucleic acidamplification technique and in situ hybridization.
 109. The method ofclaim 108, wherein said at least one probe in step (a) comprises atleast one probe that detects the expression of said nucleic acidencoding said T1R2 polypeptide by said cell and wherein said at leastone probe in step (b) comprises at least one probe that detects theexpression of said nucleic acid encoding said T1R3 polypeptide by saidcell.
 110. The method of claim 108, wherein said at least one probe instep (a) comprises at least one probe that detects the expression ofsaid nucleic acid encoding said T1R2 polypeptide by said cell.
 111. Themethod of claim 108, wherein said at least one probe in step (b)comprises at least one probe that detects the expression of said nucleicacid encoding said T1R3 polypeptide by said cell.
 112. The method ofclaim 108, wherein the nucleic acid amplification technique comprises apolymerase chain reaction amplification.
 113. The method of claim 76,further comprising using said detected cell in a high-throughputscreening assay that detects compounds that specifically activate ormodulate sweet taste signaling.
 114. The method of claim 113, whereinsaid high-throughput screening assay detects activators, inhibitors,blockers stimulators, enhancers, agonists, or antagonists of sweet tastesignaling.
 115. The method of claim 77, further comprising using saiddetected cell in a high-throughput screening assay that detectscompounds that specifically activate or modulate sweet taste signaling.116. The method of claim 76, further comprising using said detected cellin a high-throughput screening assay that detects compounds thatspecifically bind, activate or modulate the activation of a receptorcomprising said T1R2 and T1R3 polypeptides, or which modulate thebinding or activation of a receptor comprising said T1R2 and T1R3polypeptides by another compound.
 117. The method of claim 77, furthercomprising using said detected cell in a high-throughput screening assaythat detects compounds that specifically bind, activate or modulate theactivation of a receptor comprising said T1R2 and T1R3 polypeptides, orwhich modulate the binding or activation of a receptor comprising saidT1R2 and T1R3 polypeptides by another compound.
 118. The method of claim76, wherein said T1R2 polypeptide and/or said nucleic acid encoding saidT1R2 polypeptide is expressed from a nucleic acid heterologous to saiddetected cell.
 119. The method of claim 76, wherein said T1R3polypeptide and/or said nucleic acid encoding said T1R3 polypeptide isexpressed from a nucleic acid heterologous to said detected cell. 120.An in vitro method of detecting a cell that is potentially sensitive tosweet tastants, the method comprising: (a) detecting the expression of aT1R2 polypeptide and/or a nucleic acid encoding said T1R2 polypeptide bysaid cell by contacting said cell with at least one probe that detectsthe expression of said T1R2 polypeptide and/or said nucleic acidencoding said T1R2 polypeptide by said cell, wherein said T1R2polypeptide (i) is encoded by a nucleic acid sequence comprising SEQ IDNO: 10, or (ii) possesses at least 90% sequence identity to the T1R2polypeptide of SEQ ID NO: 6; (b) transfecting said cell with a T1R3polynucleotide that encodes a T1R3 polypeptide, wherein said T1R3polynucleotide (i) comprises the nucleic acid sequence of SEQ ID NO: 9,or (ii) comprises a nucleic acid sequence that encodes a polypeptidethat possesses at least 90% sequence identity to the T1R3 polypeptide ofSEQ ID NO: 7; and (c) based on the results of (a), identifying a cellthat is detected to express T1R2 as a cell that is potentially sensitiveto sweet tastants (“detected cell”).
 121. The method of claim 120,wherein step (a) occurs prior to, concurrently with, or subsequent tostep (b).
 122. The method of claim 120, further comprising confirmingthat the detected cell is sensitive to sweet tastants.
 123. The methodof claim 120, wherein the detected cell is isolated.
 124. The method ofclaim 122, wherein the detected cell is isolated.
 125. The method ofclaim 120, wherein the detected cell is isolated from a mammaliantissue.
 126. The method of claim 122, wherein the detected cell isisolated from a mammalian tissue.
 127. The method of claim 120, whereinthe detected cell is isolated from a monkey tissue.
 128. The method ofclaim 122, wherein the detected cell is isolated from a monkey tissue.129. The method of claim 120, wherein the detected cell is isolated froma human tissue.
 130. The method of claim 122, wherein the detected cellis isolated from a human tissue.
 131. The method of claim 125, whereinsaid mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 132. The method of claim126, wherein said mammalian tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 133. The method of claim127, wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 134. The method of claim128, wherein said monkey tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 135. The method of claim129, wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 136. The method of claim130, wherein said human tissue comprises tongue, oral cavity tissue,gastrointestinal epithelium, or epiglottis.
 137. The method of claim120, wherein the detected cell is isolated from a mammalian cellculture.
 138. The method of claim 122, wherein the detected cell isisolated from a mammalian cell culture.
 139. The method of claim 120,wherein the detected cell is isolated from a monkey cell culture. 140.The method of claim 122, wherein the detected cell is isolated from amonkey cell culture.
 141. The method of claim 120, wherein the detectedcell is isolated from a human cell culture.
 142. The method of claim 122wherein the detected cell is isolated from a human cell culture. 143.The method of claim 120, wherein the detected cell is transfected with aheterologous nucleic acid encoding a detectable label that provides forthe identification of compounds that specifically bind or modulate theactivity of a receptor comprising said T1R2 polypeptide and said T1R3polypeptide.
 144. The method of claim 120, wherein the detected cellendogenously expresses a G protein or is transfected with a heterologousnucleic acid encoding a G protein.
 145. The method of claim 144, whereinsaid G protein comprises gustducin, a promiscuous G protein, Gα15, Gα16,or a chimeric G protein.
 146. The method of claim 120, wherein said T1R2polypeptide and/or said nucleic acid encoding said T1R2 polypeptide isexpressed from a gene that is endogenous to said detected cell.
 147. Themethod of claim 146, wherein said detected cell is a mammalian cell.148. The method of claim 146, wherein said detected cell is a monkeycell.
 149. The method of claim 146, wherein said detected cell is ahuman cell.
 150. The method of claim 120, wherein said at least oneprobe in step (a) comprises a nucleic acid that specifically detectssaid nucleic acid encoding said T1R2 polypeptide or comprises anantibody that specifically detects said T1R2 polypeptide.
 151. Themethod of claim 150, wherein said nucleic acid encoding said T1R2polypeptide is an RNA and is detected by a method selected from thegroup consisting of Northern analysis, dot blot, RNase protection, S1analysis, a nucleic acid amplification technique and in situhybridization.
 152. The method of claim 151, wherein said at least oneprobe in step (a) comprises at least one probe that detects theexpression of said nucleic acid encoding said T1R2 polypeptide by saidcell.
 153. The method of claim 151, wherein the nucleic acidamplification technique comprises a polymerase chain reactionamplification.
 154. The method of claim 120, further comprising usingsaid detected cell in a high-throughput screening assay that detectscompounds that specifically activate or modulate sweet taste signaling.155. The method of claim 154, wherein said high-throughput screeningassay detects activators, inhibitors, blockers, stimulators, enhancers,agonists, or antagonists of sweet taste signaling.
 156. The method ofclaim 122, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallyactivate or modulate sweet taste signaling.
 157. The method of claim120, further comprising using said detected cell in a high-throughputscreening assay that detects compounds that specifically bind, activateor modulate the activation of a receptor comprising said T1R2 and T1R3polypeptides or which modulate the binding, or activation of a receptorcomprising said T1R2 and T1R3 polypeptides by another compound.
 158. Themethod of claim 122, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallybind, activate or modulate the activation of a receptor comprising saidT1R2 and T1R3 polypeptides or which modulate the binding, or activationof a receptor comprising said T1R2 and T1R3 polypeptides by anothercompound.
 159. The method of claim 120, wherein said T1R2 polypeptideand/or said nucleic acid encoding said T1R2 polypeptide is expressedfrom a nucleic acid heterologous to said detected cell.
 160. An in vitromethod of detecting a cell that is potentially sensitive to sweettastants, the method comprising: (a) transfecting said cell with a T1R2polynucleotide that encodes a T1R2 polypeptide, wherein said T1R2polynucleotide (i) comprises the nucleic acid sequence of SEQ ID NO: 10,or (ii) comprises a nucleic acid sequence that encodes a polypeptidethat possesses at least 90% sequence identity to the T1R2 polypeptide ofSEQ ID NO: 6; (b) detecting the expression of a T1R3 polypeptide and/ora nucleic acid encoding said T1R3 polypeptide by said cell by contactingsaid cell with at least one probe that detects the expression of saidT1R3 polypeptide and/or said nucleic acid encoding said T1R3 polypeptideby said cell, wherein said T1R3 polypeptide (i) is encoded by a nucleicacid sequence comprising SEQ ID NO: 9, or (ii) possesses at least 90%sequence identity to the T1R3 polypeptide of SEQ ID NO: 7; (c) based onthe results of (b), identifying a cell that is detected to express T1R3as a cell that is potentially sensitive to sweet tastants (“detectedcell”).
 161. The method of claim 160, wherein step (a) occurs prior to,concurrently with, or subsequent to step (b).
 162. The method of claim160, further comprising confirming that the detected cell is sensitiveto sweet tastants.
 163. The method of claim 160, wherein the detectedcell is isolated.
 164. The method of claim 162, wherein the detectedcell is isolated.
 165. The method of claim 160, wherein the detectedcell is isolated from a mammalian tissue.
 166. The method of claim 162,wherein the detected cell is isolated from a mammalian tissue.
 167. Themethod of claim 160, wherein the detected cell is isolated from a monkeytissue.
 168. The method of claim 162, wherein the detected cell isisolated from a monkey tissue.
 169. The method of claim 160, wherein thedetected cell is isolated from a human tissue.
 170. The method of claim162, wherein the detected cell is isolated from a human tissue.
 171. Themethod of claim 165, wherein said mammalian tissue comprises tongue,oral cavity tissue, gastrointestinal epithelium, or epiglottis.
 172. Themethod of claim 166, wherein said mammalian tissue comprises tongue,oral cavity tissue, gastrointestinal epithelium, or epiglottis.
 173. Themethod of claim 167, wherein said monkey tissue comprises tongue, oralcavity tissue, gastrointestinal epithelium, or epiglottis.
 174. Themethod of claim 168, wherein said monkey tissue comprises tongue, oralcavity tissue, gastrointestinal epithelium, or epiglottis.
 175. Themethod of claim 169, wherein said human tissue comprises tongue, oralcavity tissue, gastrointestinal epithelium, or epiglottis.
 176. Themethod of claim 170, wherein said human tissue comprises tongue, oralcavity tissue, gastrointestinal epithelium, or epiglottis.
 177. Themethod of claim 160, wherein the detected cell is isolated from amammalian cell culture.
 178. The method of claim 162, wherein thedetected cell is isolated from a mammalian cell culture.
 179. The methodof claim 160, wherein the detected cell is isolated from a monkey cellculture.
 180. The method of claim 162, wherein the detected cell isisolated from a monkey cell culture.
 181. The method of claim 160,wherein the detected cell is isolated from a human cell culture. 182.The method of claim 160, wherein the detected cell is isolated from ahuman cell culture.
 183. The method of claim 160, wherein the detectedcell is transfected with a heterologous nucleic acid encoding adetectable label that provides for the identification of compounds thatspecifically bind or modulate the activity of a receptor comprising saidT1R2 polypeptide and said T1R3 polypeptide.
 184. The method of claim160, wherein the detected cell endogenously expresses a G protein or istransfected with a heterologous nucleic acid encoding a G protein. 185.The method of claim 184, wherein said G protein comprises gustducin, apromiscuous G protein, Gα15, Gα16, or a chimeric G protein.
 186. Themethod of claim 160, wherein said T1R3 polypeptide and/or said nucleicacid encoding said T1R3 polypeptide is expressed from a gene that isendogenous to said detected cell.
 187. The method of claim 186, whereinsaid detected cell is a mammalian cell.
 188. The method of claim 186,wherein said detected cell is a monkey cell.
 189. The method of claim186, wherein said detected cell is a human cell.
 190. The method ofclaim 160, wherein said at least one probe in step (b) comprises anucleic acid that specifically detects said nucleic acid encoding saidT1R3 polypeptide or comprises an antibody that specifically detects saidT1R3 polypeptide.
 191. The method of claim 190, wherein said nucleicacid encoding said T1R3 polypeptide is an RNA and is detected by amethod selected from the group consisting of Northern analysis, dotblot, RNase protection, S1 analysis, a nucleic acid amplificationtechnique and in situ hybridization.
 192. The method of claim 191,wherein said at least one probe in step (b) comprises at least one probethat detects the expression of said nucleic acid encoding said T1R3polypeptide by said cell.
 193. The method of claim 191, wherein thenucleic acid amplification technique comprises a polymerase chainreaction amplification.
 194. The method of claim 160, further comprisingusing said detected cell in a high-throughput screening assay thatdetects compounds that specifically activate or modulate sweet tastesignaling.
 195. The method of claim 194, wherein said high-throughputscreening assay detects activators, inhibitors, blockers, stimulators,enhancers, agonists, or antagonists of sweet taste signaling.
 196. Themethod of claim 162, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallyactivate or modulate sweet taste signaling.
 197. The method of claim160, further comprising using said detected cell in a high-throughputscreening assay that detects compounds that specifically bind, activateor modulate the activation of a receptor comprising said T1R2 and T1R3polypeptides or which modulate the binding, or activation of a receptorcomprising said T1R2 and T1R3 polypeptides by another compound.
 198. Themethod of claim 162, further comprising using said detected cell in ahigh-throughput screening assay that detects compounds that specificallybind, activate or modulate the activation of a receptor comprising saidT1R2 and T1R3 polypeptides or which modulate the binding, or activationof a receptor comprising said T1R2 and T1R3 polypeptides by anothercompound.
 199. The method of claim 160, wherein said T1R3 polypeptideand/or said nucleic acid encoding said T1R3 polypeptide is expressedfrom a nucleic acid heterologous to said detected cell.